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/1.5.9.md

@@ -0,0 +1,27 @@
+---
+version: 1.5.9
+date: 2026-01-28
+---
+
+## App
+
+### New
+- UI: Files, Diff, Git, and Terminal now follow the active session/worktree directory, including new-session drafts.
+
+### Improvements
+- Worktrees: migrated to the OpenCode SDK worktree implementation; sessions in worktrees are now completely isolated.
+- Git: integrate worktree commits back to a target branch with commit previews and guided conflict handling.
+- Files: toggle markdown preview when viewing files (thanks to @Jovines).
+- Files: open the file viewer in fullscreen for focused review and editing (thanks to @TaylorBeeston).
+- Plans: switch between markdown preview and edit mode in the Plan view.
+
+### Fixes
+- Web: plan lists no longer error when the plans directory is missing.
+
+## VS Code
+
+### Improvements
+- Agent Manager: migrated to the OpenCode SDK worktree implementation; sessions in worktrees are now completely isolated.
+
+### Fixes
+- Agent Manager: worktree setup commands are now persistent per project and automatically saved/restored.