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/1.13.6.md

@@ -0,0 +1,22 @@
+---
+version: 1.13.6
+date: 2026-06-28
+---
+
+## App
+
+### New
+- Desktop/macOS: the Dock icon can now show a badge count for chats with unseen activity, with a new Appearance setting to turn it off.
+
+### Improvements
+- Chat: scrolling in conversations now stays steady while sending, queueing, streaming, switching sessions, and loading older messages.
+- Chat: selecting a user-installed skill from the slash command menu now invokes the skill and injects its content, instead of inserting the skill name as plain text.
+- Context Panel: chat tabs now use the session title and mark the open chat as seen while you are viewing it.
+
+### Fixes
+- Context Panel: Browser and Preview tabs no longer accumulate duplicate auth tokens in their URLs after reloads or navigation.
+
+## VS Code
+
+### Improvements
+- Chat: scrolling in conversations now stays steady while sending, queueing, streaming, switching sessions, and loading older messages.