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/1.5.0.md

@@ -0,0 +1,39 @@
+---
+version: 1.5.0
+date: 2026-01-16
+---
+
+## App
+
+### New
+- UI: added a new Files tab to browse workspace files directly from the interface.
+- Git Identities: added "default identity" setting with one-click set/unset and automatic local identity detection.
+- VSCode: added responsive layout with sessions sidebar + chat side-by-side when wide, compact header, and streamlined settings.
+
+### Improvements
+- Diff: enhanced the diff viewer with mobile support and the ability to ask the agent for comments on changes.
+- VSCode: improved server management to ensure it initializes within the workspace directory with context-aware readiness checks.
+- Web: the server now automatically resolves and uses an available port if the default is occupied.
+
+### Fixes
+- Web/VSCode: fixed orphaned OpenCode processes not being cleaned up on restart or exit.
+- Stability: fixed heartbeat race condition causing session stalls during long tasks (thanks to @tybradle).
+- Desktop: fixed commands for worktree setup access to PATH.
+
+## VS Code
+
+### New
+- Layout: added responsive expanded layout showing sessions sidebar + chat side-by-side when extension is wide enough (≥700px).
+- Layout: extension now opens to sessions list instead of new session draft.
+
+### Improvements
+- Improved OpenCode server management to ensure it initializes within the workspace directory.
+- Enhanced extension startup with context-aware readiness checks for the current workspace.
+- Layout: compact header with reduced padding.
+- Settings: hidden Git Identities tab, Git section, and Diff view settings (not applicable to VS Code).
+- Settings: hidden project switcher dropdown (VS Code uses workspace).
+- Shortcuts: disabled worktree session creation with shortcuts (Ctrl+Shift+N now opens standard session).
+
+### Fixes
+- Fixed orphaned OpenCode processes not being cleaned up on restart or exit.
+- Session tabs: fixed opening new session in editor tab; title bar button now opens new session tab, sidebar button opens current or new session.