feat: advertise and serve agent skills from .well-known (#44878)

## Summary

- Adds `apps/www/scripts/fetchAgentSkills.mjs` — at build time (`pnpm
content:build`) fetches the latest `index.json` from
`supabase/agent-skills` release assets and writes it verbatim to
`public/.well-known/agent-skills/index.json`
- Skill URLs are absolute GitHub Release asset URLs, embedded by the
agent-skills repo at release time (supabase/agent-skills#87) — no URL
rewriting needed on this side

## How it works

1. Fetches latest release from `supabase/agent-skills` via GitHub API
2. Downloads `index.json` from the release assets
3. Writes it verbatim to `public/.well-known/agent-skills/index.json`

Clients discover and install skills from
`supabase.com/.well-known/agent-skills/index.json` and fetch tarballs
directly from GitHub Release assets.

## Dependency

Requires supabase/agent-skills#87 to be merged and released so the
published `index.json` contains absolute URLs.

---------

Co-authored-by: Pedro Rodrigues <44656907+Rodriguespn@users.noreply.github.com>
Co-authored-by: Claude Sonnet 4.6 (1M context) <noreply@anthropic.com>
Co-authored-by: Pedro Rodrigues <pedro.rodrigues@supabase.io>

apps/www/package.json

@@ -13,7 +13,7 @@
     "clean": "rimraf node_modules",
     "pretypecheck": "next typegen",
     "typecheck": "pnpm run content:build && tsc --noEmit",
-    "content:build": "node scripts/generateStaticContent.mjs && node scripts/generateMdContent.mjs",
+    "content:build": "node scripts/generateStaticContent.mjs && node scripts/generateMdContent.mjs && node scripts/fetchAgentSkills.mjs",
     "test": "vitest --run",
     "test:watch": "vitest watch",
     "postbuild": "node ./internals/generate-sitemap.mjs && ./../../scripts/upload-static-assets.sh"

apps/www/scripts/fetchAgentSkills.mjs

@@ -0,0 +1,49 @@
+// @ts-check
+
+/**
+ * Fetches the latest agent-skills index.json from supabase/agent-skills and
+ * writes it to public/.well-known/agent-skills/index.json.
+ *
+ * Skill URLs in the published index.json are absolute GitHub Release asset
+ * URLs — no rewriting needed on this side.
+ *
+ * Spec: https://github.com/agentskills/agentskills/pull/254
+ * Runs unauthenticated — public repo, build-time only.
+ */
+
+import { promises as fs } from 'node:fs'
+import { dirname, join } from 'node:path'
+import { fileURLToPath } from 'node:url'
+
+const __dirname = dirname(fileURLToPath(import.meta.url))
+const OUT_DIR = join(__dirname, '..', 'public', '.well-known', 'agent-skills')
+const REPO = 'supabase/agent-skills'
+
+async function fetchJson(url) {
+  const res = await fetch(url, { headers: { 'User-Agent': 'supabase-www-build' } })
+  if (!res.ok) throw new Error(`GET ${url} → ${res.status}`)
+  return res.json()
+}
+
+async function main() {
+  const release = await fetchJson(`https://api.github.com/repos/${REPO}/releases/latest`)
+  console.log(`Fetching agent-skills release: ${release.tag_name}`)
+
+  const indexAsset = release.assets.find((a) => a.name === 'index.json')
+  if (!indexAsset) throw new Error('No index.json found in release assets')
+
+  const index = await fetchJson(indexAsset.browser_download_url)
+
+  await fs.mkdir(OUT_DIR, { recursive: true })
+  await fs.writeFile(join(OUT_DIR, 'index.json'), JSON.stringify(index, null, 2) + '\n')
+
+  for (const skill of index.skills ?? []) {
+    console.log(`  ${skill.name}`)
+  }
+  console.log(`Done — wrote public/.well-known/agent-skills/index.json`)
+}
+
+main().catch((err) => {
+  console.error(err)
+  process.exit(1)
+})