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/1.6.0.md

@@ -0,0 +1,31 @@
+---
+version: 1.6.0
+date: 2026-01-29
+---
+
+## App
+
+### New
+- Chat: added message stall detection with automatic soft resync.
+
+### Improvements
+- Git: PR picker now validates local branch existence and includes a refresh action.
+- Git: worktree integration now syncs clean target directories before merging.
+- VSCode: session activity status now updates reliably even when the webview is hidden.
+- Web: session activity tracking now works consistently across browser tabs.
+
+### Fixes
+- Chat: fixed "Load older" button behavior in chat with proper pagination implementation.
+- Diff: fixed memory leak when viewing many modified files; large changesets now lazy-load for smooth performance.
+- Reliability: plans directory no longer errors when missing.
+
+## VS Code
+
+### New
+- Added message stall detection with automatic soft resync.
+
+### Improvements
+- Session activity status now updates reliably even when the extension panel is hidden or collapsed.
+
+### Fixes
+- Fixed "Load older" button in long sessions with proper progressive pagination.