fix(www): avoid fetching agent skills during development (#51111)

## Problem

`pnpm dev:www` ran `content:build`, whose final `fetchAgentSkills` step
rewrote the committed skills index.

## Change

Local development now runs the existing `content:build:core` generators
and serves the committed index. Every WWW build still runs
`content:build`, which runs the core generators followed by
`fetchAgentSkills`.

```mermaid
flowchart LR
  L[Local dev] --> C[content:build:core]
  C --> N[Next dev]
  I[Committed index] --> N
  B[Preview / production build] --> F[content:build]
  F --> C
  F --> A[fetchAgentSkills]
  R[Latest GitHub release] --> A
  A --> D[Next build]
```

Preview builds fall back to the committed index if fetching fails.
Production builds fail rather than publish a stale index. The committed
fallback is updated to v0.1.9.

## Test plan

- `pnpm dev:www` leaves the index unchanged and serves it byte-for-byte
- stale v0.1.8 fixture refreshes to v0.1.9 through the real fetch script
- `pnpm --filter www test turbo-build.test.ts`
- `pnpm --filter www typecheck`
- `pnpm exec prettier --check apps/www/package.json
apps/www/public/.well-known/agent-skills/index.json`

- [x] I have read
[CONTRIBUTING.md](https://github.com/supabase/supabase/blob/master/CONTRIBUTING.md)

Closes
[AI-1275](https://linear.app/supabase/issue/AI-1275/running-pnpm-devwww-modifies-the-generated-agent-skills-index)

Co-authored-by: OpenAI Codex <noreply@openai.com>

apps/www/package.json

@@ -5,7 +5,7 @@
   "private": true,
   "scripts": {
     "preinstall": "npx only-allow pnpm",
-    "dev": "pnpm run content:build && next --port 3000",
+    "dev": "pnpm run content:build:core && next --port 3000",
     "prebuild": "pnpm --filter=docs run build:federated-content && pnpm --filter=docs run build:guides-markdown && pnpm run content:build",
     "build": "turbo run build:next --filter=www",
     "build:next": "next build && node ./internals/generate-sitemap.mjs",

apps/www/public/.well-known/agent-skills/index.json

@@ -5,14 +5,14 @@
       "name": "supabase",
       "type": "archive",
       "description": "Use when doing ANY task involving Supabase. Triggers: Supabase products (Database, Auth, Edge Functions, Realtime, Storage, Vectors, Cron, Queues); client libraries and SSR integrations (supabase-js, @supabase/ssr) in Next.js, React, SvelteKit, Astro, Remix; auth issues (login, logout, sessions, JWT, cookies, getSession, getUser, getClaims, RLS); Supabase CLI or MCP server; schema changes, migrations, declarative schemas, security audits, Postgres extensions (pg_graphql, pg_cron, pg_vector); debugging and troubleshooting errors or unexpected behavior on Supabase projects (HTTP errors, Postgres errors, RLS surprises, permission denied, schema cache issues, timeouts, Edge Function crashes, Realtime drops, Storage failures) and reading or querying logs (Logs Explorer, ClickHouse).",
-      "url": "https://github.com/supabase/agent-skills/releases/download/v0.1.8/supabase.tar.gz",
-      "digest": "sha256:d90b81c17612ee62372e47f628cc20056ab1a3cb1711780511cefc41113b86ad"
+      "url": "https://github.com/supabase/agent-skills/releases/download/v0.1.9/supabase.tar.gz",
+      "digest": "sha256:52069676431ed41025b74e0b8807d908c08c10d5cdb842415684c47041016fbf"
     },
     {
       "name": "supabase-postgres-best-practices",
       "type": "archive",
       "description": "Postgres best practices maintained by Supabase, for Postgres running anywhere. Load this skill BEFORE writing or changing anything that lives in a Postgres database: creating or altering tables and columns (including choosing column types), schema design, migrations and declarative schema files, RLS policies and the tests that verify them, indexes, triggers, database functions, queues and scheduled jobs (pg_cron, pgmq), vector/semantic search (pgvector), and restoring dumps (pg_restore) or importing data. Also load it when diagnosing slow queries, high CPU, timeouts, EXPLAIN plans, connection exhaustion, locking, bloat, or rows visible to the wrong user or tenant. This is not just a performance guide — schema, migration, security, and SQL authoring tasks need these rules too, even for a one-column change or a single query.",
-      "url": "https://github.com/supabase/agent-skills/releases/download/v0.1.8/supabase-postgres-best-practices.tar.gz",
+      "url": "https://github.com/supabase/agent-skills/releases/download/v0.1.9/supabase-postgres-best-practices.tar.gz",
       "digest": "sha256:b7fc538bc4319134b295f2e8c01f9b1ccca7e63e4666f11b81de8b7f944cf187"
     }
   ]