Every workflow was broken after the Fumadocs migration and the Node 25
Docusaurus breakage. Fixing in one pass.

Problems that were live on main:

- ci.yml, coverage.yml, size.yml, release.yml all ran 'pnpm build'
  (no filter). Turbo tries to build every workspace project including
  @agentskit/docs, which is Docusaurus-on-Node-25 and crashes on
  'mdxCrossCompilerCache'. Every one of these jobs failed in CI.

- docs.yml still deployed @agentskit/docs (old Docusaurus) to GitHub
  Pages. That site is a) retired, b) doesn't build anyway. The
  replacement lives at apps/docs-next (Fumadocs) and deploys via
  Vercel's GitHub integration, not GitHub Pages.

- ci.yml duplicated bundle-size enforcement by hand-rolling a
  check_size bash function — size.yml already does this properly
  via size-limit with the .size-limit.json budget. Paths were wrong
  too (no packages/react/dist/index.js without the preceding build).

- Version drift across workflows: checkout@v4 vs v6, pnpm@v4 vs v5,
  node 20 vs 22. doctor's recommendation is Node 22+.

What this does:

1. Standardize every workflow to:
     actions/checkout@v4
     pnpm/action-setup@v4
     actions/setup-node@v4
     node-version: 22

2. Replace every 'pnpm build' with 'pnpm --filter "./packages/*" build'.
   Legacy docs is not in scope for CI until it's retired.

3. ci.yml — drop the hand-rolled size check. size.yml is the single
   source of truth for bundle-size gates.

4. docs.yml — convert from 'deploy Docusaurus to GH Pages' to
   'build @agentskit/docs-next on every relevant PR/push'. Deploy
   is Vercel's job via the repo's GitHub App integration. Workflow
   is now validation-only + path-filtered so it doesn't run for
   unrelated changes.

5. release.yml — same filter fix, same version bumps. Changesets
   action itself is unchanged.

No i18n wiring — per the i18n strategy RFC, translated content is
frozen during the Fumadocs migration, so the workflow has nothing
to do for locales.

Verified locally:
- pnpm --filter './packages/*' lint → 28/28 passing
- pnpm --filter './packages/*' build → 22/22 passing
- pnpm test → 22/22 passing, 538 tests
- pnpm exec size-limit → every package under budget
- pnpm --filter @agentskit/docs-next build → 74 routes clean

Refs #211

…deps