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/1.5.3.md

@@ -0,0 +1,30 @@
+---
+version: 1.5.3
+date: 2026-01-20
+---
+
+## App
+
+### New
+- Settings: new memory limits controls for session message history.
+- Chat: new Activity view in collapsed state, now shows latest 6 tools by default.
+
+### Improvements
+- Files: edit files inline with syntax highlighting, draft protection, and save/discard flow.
+- Files: toggles to show hidden/dotfiles and gitignored entries in file browsers and pickers (thanks to @syntext).
+- Chat: smooth session switching with more stable scroll anchoring.
+- Git: generated commit messages now auto-pick a gitmoji when enabled (thanks to @TheRealAshik).
+- Performance: faster filesystem/search operations and general stability improvements (thanks to @TheRealAshik).
+
+### Fixes
+- Chat: fixed message copy on Firefox for macOS (thanks to @syntext).
+- Appearance: new corner radius control and restored input bar offset setting (thanks to @TheRealAshik).
+
+## VS Code
+
+### Improvements
+- Chat: improved session switching with more stable scroll anchoring.
+- Chat: the collapsed Activity view now shows the latest 6 tools by default.
+- Chat: updated accent color derivation to better match editor themes.
+- Performance: improved filesystem/search speed and general stability (thanks to @TheRealAshik).
+- Files: adjusted default visibility for hidden/dotfiles to be visible and gitignored entries to be hidden.