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/1.3.8.md

@@ -0,0 +1,31 @@
+---
+version: 1.3.8
+date: 2025-12-29
+---
+
+## App
+
+### New
+- Added Intel Mac (x86_64) support for the desktop application (thanks to @rothnic).
+- Added queued message mode with chips, batching, and idle auto‑send (including attachments).
+- Added queue mode toggle to OpenChamber settings (chat section) with persistence across runtimes.
+
+### Improvements
+- Build workflow now generates separate builds for Apple Silicon (arm64) and Intel (x86_64) Macs (thanks to @rothnic).
+- Improved dev server HMR by reusing a healthy OpenCode process to avoid zombie instances.
+- Refactored Agents/Commands management with ability to configure project/user scopes.
+
+### Fixes
+- Fixed scroll position persistence for active conversation turns across session switches.
+
+## VS Code
+
+### New
+- Added queued message mode with chips, batching, and idle auto‑send (including attachments).
+- Added queue mode toggle to settings (chat section).
+
+### Improvements
+- Refactored Agents/Commands management with ability to configure project/user scopes.
+
+### Fixes
+- Fixed scroll position persistence for active conversation turns across session switches.