ci(changelog): open changelog PR from platform-release dispatch

[atharva-bhange]

What changed

Adds the docs-side of the release-automation fan-out (Track F): a changelog-sync
GitHub Actions workflow that consumes the platform-release repository_dispatch
event and opens an editorial PR appending the new version section to the public
changelog page.

• .github/workflows/changelog-sync.yml — receives repository_dispatch
  (types: [platform-release]), base64-decodes release_body_b64, runs the
  transform script against src/pages/changelog.mdx, then commits to a
  chore/changelog-<version> branch and opens a PR to main.
• scripts/changelog-from-release.mjs — dependency-free (Node ≥20) transform
  that maps release-please sections (Features→New Features,
  Bug Fixes, Performance Improvements→Improvements, Reverts,
  BREAKING CHANGES→Breaking Changes), drops unmapped sections (e.g. Chores),
  and inserts the generated section immediately below the marker.
• scripts/changelog-from-release.test.mjs — node --test unit tests for
  transform and insert.
• src/pages/changelog.mdx — inserts the insertion marker comment only
  ({/* changelog:insert-below … */}). Placeholder entries are intentionally
  left in place (a later human/editorial decision, Track G).

Why

Implements the docs changelog sync described in RELEASE-PROCESS-PLAN.md §5.4:
when future-agi cuts a platform release, the docs repo receives a dispatch and
raises an editorial PR so a human can rewrite raw commit bullets for a product
audience before publishing.

How verified

node --test scripts/changelog-from-release.test.mjs:

✔ transform maps release-please sections to changelog sections
✔ insert places section after marker and preserves the rest
✔ insert throws when marker missing
ℹ tests 3
ℹ pass 3
ℹ fail 0

End-to-end insert against the real src/pages/changelog.mdx placed
## v9.9.9 - <current month> on the line directly below the marker (above the
placeholder v2.0.0 entry), then restored the file.

actionlint .github/workflows/changelog-sync.yml → exit 0, no findings.
(Repo-root actionlint also reports pre-existing SC20xx findings in
deploy.yml, which this PR does not touch.)

Follow-ups

• Placeholder changelog entries cleanup is deferred to Track G (human editorial).
• The workflow depends on the org-level platform-release dispatch being wired
  from future-agi (Track D) and the RELEASE_BOT_TOKEN secret (Phase 0).

🤖 Generated with Claude Code

[atharva-bhange]

Closing: process change — changes will be reviewed locally before committing; a fresh PR will follow after sign-off.