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/1.6.8.md

@@ -0,0 +1,33 @@
+---
+version: 1.6.8
+date: 2026-02-12
+---
+
+## App
+
+### New
+- Chat: added drag-and-drop attachments with inline image previews.
+
+### Improvements
+- Chat: improved agent/model picking with fuzzy search across names and descriptions.
+- Usage: corrected Gemini and Antigravity quota source mapping and labels (thanks to @gsxdsm).
+- Usage: when using remaining-quota mode, usage markers now invert direction to better match how remaining capacity is interpreted (thanks to @gsxdsm).
+
+### Fixes
+- Sessions: fixed a sidebar issue where draft input could carry over when switching projects.
+- Chat: improved quick navigation from the sessions list by adding double-click to jump into chat and auto-focus the draft input; also fixed mobile session return behavior (thanks to @gsxdsm).
+- Desktop: fixed project selection in opened remote instances.
+- Desktop: fixed opened remote instances that use HTTP (helpful for instances under tunneling).
+
+## VS Code
+
+### New
+- Chat: added drag-and-drop attachments with inline image previews.
+
+### Improvements
+- Chat: improved picker search with fuzzy matching on names and descriptions to speed up finding the right agent/model.
+- Usage: corrected Gemini and Antigravity quota source mapping and labels (thanks to @gsxdsm).
+- Usage: remaining-quota mode now inverts usage markers (thanks to @gsxdsm).
+
+### Fixes
+- Sessions: fixed previously selected session carry-over when navigating from chat / session draft and list of sessions.