chore(studio): gate dead code with knip in CI (#49721)

Makes knip a CI gate for Studio so dead files and unused dependencies
fail the PR instead of piling up. Third PR in the stack, on top of
#49719 (dead code) and #49720 (unused deps), which get Studio to a clean
run.

**Changed:**
- knip `pnpx knip@~5.50.0` → root devDependency `knip@6.32.3`, `pnpm
knip` now runs it. The old `pnpx` form was actually broken: it resolved
knip's `typescript` peer to TS 7 and crashed with
`ts.getDefaultLibFilePath is not a function`. (6.33.0 is newer but
blocked by `minimumReleaseAge`.)
- `knip.jsonc` rewritten for v6 with a `workspaces["apps/studio"]`
block. Framework-convention files (`router.tsx`, `start.ts`,
`routes/**`, `compat/**`, `api/server.js`) are `entry` rather than
`ignore` — an ignored file's imports aren't traced, which is how
`ShellFallback.tsx` (only imported from `routes/__root.tsx`) was being
reported as dead. knip 6's Next.js plugin already covers
`instrumentation*.ts`, `proxy.ts`, `pages/**`; its tanstack-router
plugin only looks under `src/`, hence the manual entries. Narrow
`ignoreIssues` for graphql-codegen output and the `CONSTRAINT_TYPE`
enum; `ignoreDependencies` for the five implicit deps from #49720, each
with a comment; `ignoreBinaries: ["vercel"]`.
- `apps/studio/CLAUDE.md`: one bullet on the gate and where framework
files go.

**Added:**
- `.github/workflows/studio-knip.yml` — path-filtered to
`apps/studio/**` + knip/pnpm config, mirrors `studio-lint-ratchet.yml`'s
setup (no sparse checkout: knip needs every workspace's `package.json`
to resolve the graph). Runs `pnpm knip --workspace apps/studio
--reporter symbols --reporter github-actions` so findings show up as
inline PR annotations. ~5s locally.

Scope notes: the gate is Studio-only — the full-monorepo run still has
~400 dead files in `www`/`docs`/`blocks`, which is a separate effort.
`exclude: ["types", "exports"]` is kept, so unused exports aren't gated
yet, but `enumMembers`/`duplicates` are (they caught real things in
#49719).

## To test

- `pnpm knip --workspace apps/studio` exits 0 on this branch
- The `Studio Dead Code (knip)` workflow runs on this PR and is green
- Sanity-check the gate bites: add a throwaway
`apps/studio/lib/unused.ts`, run `pnpm knip --workspace apps/studio` →
reports it and exits 1


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

* **CI**
* Added automated dead-code and unused-dependency checks for the Studio
workspace on relevant pushes and pull requests.
* Results appear in workflow summaries and as inline pull request
annotations.

* **Maintenance**
  * Improved analysis of framework-convention files and Studio code.
* Standardized the local code-quality check and updated its
configuration support.
<!-- end of auto-generated comment: release notes by coderabbit.ai -->

---------

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

package.json

@@ -48,7 +48,7 @@
     "setup:cli": "supabase start -x studio && supabase status --output json > keys.json && node scripts/generateLocalEnv.js",
     "generate:types": "supabase gen types typescript --local > ./supabase/functions/common/database-types.ts",
     "api:codegen": "cd packages/api-types && pnpm run codegen",
-    "knip": "pnpx knip@~5.50.0",
+    "knip": "knip",
     "authorize-vercel-deploys": "tsx scripts/authorizeVercelDeploys.ts"
   },
   "devDependencies": {
@@ -57,6 +57,7 @@
     "@types/node": "catalog:",
     "eslint": "^9.0.0",
     "jsdom": "^28.1.0",
+    "knip": "6.32.3",
     "prettier": "^3.8.0",
     "prettier-plugin-sql-cst": "^0.18.0",
     "rimraf": "^6.0.0",