feat(changelog): one source file per release, generated outputs

Release notes now live in changelog/<version>.md (front matter with
version and date, then ## App and ## VS Code sections grouped into
New, Improvements, Fixes, Misc) plus changelog/unreleased.md for what
has not shipped. `bun run changelog:build` renders CHANGELOG.md,
packages/vscode/CHANGELOG.md, and changelog/index.json from them;
`changelog:check` fails when the outputs are behind and runs in CI and
in release:prepare. `oc-dev create-release` promotes unreleased.md to
the versioned file dated today and rebuilds.

The existing history was split mechanically: every bullet kept, sorted
into groups by keyword, five hand-typed headers with one-digit days
normalised to YYYY-MM-DD (the update dialog matched none of them). The
generated files keep today's release headers, which the update dialog,
the release workflow, and the website match by regex.

Claude-Session: https://claude.ai/code/session_01VqV56Hez25hTxXH4ipJfzH

changelog/1.6.3.md

@@ -0,0 +1,22 @@
+---
+version: 1.6.3
+date: 2026-02-02
+---
+
+## App
+
+### Improvements
+- Web: improved server readiness check to use the `/global/health` endpoint.
+- VSCode: improved server health check with the proper health API endpoint and increased timeout for steady startup (thanks to @wienans).
+
+### Fixes
+- Web: added login rate limit protection to prevent brute-force attempts on the authentication endpoint (thanks to @Jovines).
+- Settings: dialog no longer persists open/closed state across app restarts.
+
+## VS Code
+
+### Improvements
+- Improved server health check with the proper health API endpoint and increased timeout for steady startup (thanks to @wienans).
+
+### Fixes
+- Settings dialog no longer persists open/closed state across extension restarts.