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/1.3.0.md

@@ -0,0 +1,33 @@
+---
+version: 1.3.0
+date: 2025-12-21
+---
+
+## App
+
+### New
+- Added revert functionality in chat for user messages.
+
+### Improvements
+- Polished mobile controls in chat view.
+- Updated user message layout/styling.
+- Improved header tab responsiveness.
+- Adjusted VSCode extension theme mapping and model selection view.
+- Polished file autocomplete experience.
+
+### Fixes
+- Fixed bugs with new session creation when the VSCode extension initialized for the first time.
+
+## VS Code
+
+### New
+- Added revert functionality in chat for user messages.
+
+### Improvements
+- Updated user message layout/styling.
+- Improved header tab responsiveness.
+- Adjusted extension theme mapping and model selection view.
+- Polished file autocomplete experience.
+
+### Fixes
+- Fixed bugs with new session creation when the extension initialized for the first time.