chore: make agent instructions agent-agnostic (#49941)

Makes the repo's AI-agent setup tool-agnostic: instructions live in
`AGENTS.md` files, skills live in `.agents/skills/`, and Claude Code,
Codex, Cursor, and Copilot all read the same sources. Also sweeps the
skills for stale and duplicated content while everything was being
moved.

**Changed:**
- Every `CLAUDE.md` (root, `apps/studio`, `apps/docs`, `apps/kb`) is now
a one-line `@AGENTS.md` import; the content moved verbatim into an
`AGENTS.md` beside it. The root one moved from `.claude/CLAUDE.md` to
the repo root for consistency.
- All skills now live in `.agents/skills/`; `.claude/skills` is a single
symlink to it (replacing the old mix of real dirs and per-skill
symlinks). Path references in `.coderabbit.yaml`, code comments, and
docs updated to match.
- `.github/copilot-instructions.md` keeps only the review policy and
points at `AGENTS.md` + `.agents/skills/`. Copilot code review reads
those natively now, so the per-topic
`.github/instructions/*.instructions.md` files were duplicates of the
skills.
- Stale skill content fixed: `studio-queries` imported a toast library
Studio doesn't use, `telemetry-standards` and `studio-testing` used
import paths that don't resolve, `safe-sql-execution` cited a boundary
test that doesn't exist, the ask-the-docs references described an
`AiPrompt` mechanism that was replaced by the ID-keyed registry, plus a
handful of wrong paths, a self-contradicting `waitForTimeout` rule, an
invalid Playwright signature, and a ConfigCat flag described as PostHog.
- `studio-error-handling` now explains when to use `AlertError` (the
default) vs `ErrorMatcher`.

**Added:**
- `apps/docs/AGENTS.md` (docs test requirements, from the old Cursor
rule)
- `studio-shortcuts` skill (from the old Copilot instruction file,
verified against the current registry)
- `ask-the-docs/reference/graphql-endpoint.md` and
`search-embeddings.md` (from the old Cursor rules, with the missing
resolver/registration/codegen steps filled in)
- Feature-flag measurement section in `telemetry-standards`

**Removed:**
- `.cursor/` (rules folded in as above; skill symlinks no longer needed)
and `.cursorignore`
- `.github/instructions/` (8 files)
- `vercel-composition-patterns/AGENTS.md` – a 946-line verbatim
concatenation of its own `rules/` directory, and a nested `AGENTS.md`
that agents could auto-load as repo instructions
- `edit-the-docs/reference/structure-and-flow.md` – word-for-word copy
of the skill's own Phase 2 text

## To test

- `readlink .claude/skills` → `../.agents/skills`, and `ls
.claude/skills/copywriting/SKILL.md` resolves
- Open a Claude Code session at the repo root and in `apps/studio` – the
imported `AGENTS.md` content should load as before
- `git diff master --stat -M` shows the skill moves as 100% renames
(content unchanged except the listed fixes)
- Spot-check a fixed claim, e.g. `import { toast } from 'sonner'` in
`studio-queries`, or the `logs.all` ESLint rule cited in
`clickhouse-logs-queries/references/codebase-integration.md`

<!-- This is an auto-generated comment: release notes by coderabbit.ai
-->
## Summary by CodeRabbit

- **Documentation**
- Expanded guidance for documentation workflows, GraphQL resources,
search, ClickHouse logs, React forms, Studio testing, shortcuts,
telemetry, accessibility, copywriting, and composition patterns.
- Clarified local testing, linting, build workflows, error handling, and
AI coding agent usage.
- Added contributor guidance for the knowledge base, documentation, and
Studio areas.

- **Chores**
  - Consolidated agent instructions and skill references.
- Removed obsolete editor-specific guidance, duplicate links, and
superseded documentation.
<!-- end of auto-generated comment: release notes by coderabbit.ai -->

---------

Co-authored-by: Alaister Young <10985857+alaister@users.noreply.github.com>

.github/copilot-instructions.md

@@ -19,7 +19,7 @@ Our CI pipeline already validates the following. **Never comment on these topics
 - **Missing tests for trivial changes** — Handled by topic-specific test instructions
 - **Import ordering or grouping** — Handled by linter
 - **Naming style preferences** (camelCase vs snake_case debates) — Follow existing file conventions
-- **Accessibility attributes on shadcn/Radix UI components** — See `studio-shadcn-components.instructions.md` for details
+- **Accessibility attributes on shadcn/Radix UI components** — handled by the primitives; see [shadcn/Radix accessibility](#shadcnradix-accessibility) below
 
 ### What TO Comment On (Priority Order)
 
@@ -47,15 +47,41 @@ This is a TypeScript/Next.js/React monorepo:
 
 ## Topic-Specific Guidelines
 
-Path-specific rules in `.github/instructions/`:
+Coding conventions are not duplicated here. Read them from the shared agent instruction files, which apply to every AI tool working in this repo:
 
-- **Telemetry**: `studio-telemetry.instructions.md` — event naming, property conventions, feature flag measurement
-- **Testing**: `studio-testing.instructions.md` — test strategy, extraction patterns, coverage expectations
-- **Error Handling**: `studio-error-handling.instructions.md` — error classification, `ErrorMatcher` usage
-- **E2E Tests**: `studio-e2e-tests.instructions.md` — selector priority, anti-patterns (`waitForTimeout`, `force: true`)
-- **Composition Patterns**: `studio-composition-patterns.instructions.md` — avoid boolean props, use compound components
-- **UI Copy**: `studio-copy.instructions.md` → `apps/design-system/content/docs/copywriting.mdx`
-- **shadcn/Radix Components**: `studio-shadcn-components.instructions.md` — accessibility handled by primitives, do not flag
-- **Keyboard Shortcuts**: `studio-shortcuts.instructions.md` — shortcut registry pattern, search-input escape handler, when to flag missing coverage
+- `AGENTS.md` (repo root) — monorepo structure, commands, CI, conventions
+- `apps/studio/AGENTS.md` — Studio-specific rules and the task → skill map
+- `.agents/skills/*/SKILL.md` — the source of truth per topic. Most relevant to review: `studio-testing`, `studio-mock-api-tests`, `studio-e2e-tests`, `studio-error-handling`, `studio-queries`, `studio-ui-patterns`, `react-hook-form`, `telemetry-standards`, `safe-sql-execution`, `vercel-composition-patterns`, `studio-shortcuts`, `copywriting`
 
-These files are scoped to `apps/studio/` and applied automatically during reviews.
+When a skill says to flag something, treat it as **advisory** here — the confidence threshold and comment style above still apply.
+
+## shadcn/Radix accessibility
+
+Studio uses **shadcn/ui** components built on **Radix UI** primitives (from `packages/ui/`), which provide ARIA roles, keyboard navigation, focus management, and screen-reader support automatically. **Do not flag missing accessibility attributes that the underlying primitive already handles.**
+
+| Component                     | What Radix handles                                                               |
+| ----------------------------- | -------------------------------------------------------------------------------- |
+| `Dialog`, `AlertDialog`       | `role="dialog"`, `aria-modal`, focus trapping, ESC to close                      |
+| `DropdownMenu`, `ContextMenu` | `role="menu"` / `role="menuitem"`, arrow key navigation                          |
+| `Select`                      | `role="combobox"`, `aria-expanded`, keyboard selection                           |
+| `Tabs`                        | `role="tablist"` / `role="tab"` / `role="tabpanel"`, `aria-selected`, arrow keys |
+| `Checkbox`                    | `role="checkbox"`, `aria-checked`, Space to toggle                               |
+| `RadioGroup`                  | `role="radio"`, `aria-checked`, arrow key navigation                             |
+| `Switch`                      | `role="switch"`, `aria-checked`, keyboard toggle                                 |
+| `Tooltip`                     | Trigger/content association, show/hide timing                                    |
+| `Accordion`, `Collapsible`    | `aria-expanded`, Enter/Space to toggle                                           |
+| `Popover`, `HoverCard`        | Focus management, dismiss on ESC                                                 |
+| `Slider`                      | `role="slider"`, `aria-valuemin/max/now`, arrow keys                             |
+| `Toggle`, `ToggleGroup`       | `aria-pressed`, keyboard support                                                 |
+| `ScrollArea`                  | Accessible scrollbar replacement                                                 |
+| `NavigationMenu`              | `role="navigation"`, keyboard navigation                                         |
+
+Specifically, never flag: missing `role` on Radix-based components; missing `aria-modal` on `Dialog`/`AlertDialog`; missing `aria-expanded` on `Accordion`, `Collapsible`, `Select`, or `DropdownMenu` triggers; missing `aria-selected` on `Tabs`; missing `aria-checked` on `Checkbox`, `RadioGroup`, or `Switch`; missing keyboard handlers on interactive Radix components; missing focus management in dialogs; missing `aria-label` on `DialogClose` (it renders `<span className="sr-only">Close</span>`).
+
+**Do** flag accessibility issues on:
+
+1. **Custom interactive elements** not using Radix primitives (e.g. a `<div onClick>` that should be a `<button>`)
+2. **Icon-only buttons** missing an accessible label — `<Button>` alone does not add one; use `aria-label` or `<span className="sr-only">`
+3. **Missing `Label` association** — form inputs should be paired with `<Label htmlFor="...">` or wrapped in a `<Field>` component
+4. **Images missing `alt` text**
+5. **Color-only state indicators**