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/1.12.2.md

@@ -0,0 +1,23 @@
+---
+version: 1.12.2
+date: 2026-06-05
+---
+
+## App
+
+### New
+- **Desktop/Windows: the Windows app is now available publicly, with full functionality parity across the app.**
+- Projects: the Add Project directory picker now starts with hidden files off each time it opens.
+
+### Improvements
+- Tunnels: switching between Cloudflare and ngrok quick tunnels now replaces the active quick tunnel instead of reusing the previous provider.
+- Tunnels: ngrok startup failures now show the ngrok or authtoken error returned during startup.
+- Chat: prompts sent while creating or switching target sessions now stay attached to the intended project directory.
+- VSCode: the extension now detects more Windows OpenCode installs from PATH, npm, Scoop, and Chocolatey.
+
+## VS Code
+
+### Improvements
+- Startup/Windows: the extension now detects more OpenCode installs from PATH, npm, Scoop, and Chocolatey.
+- Chat: prompts sent while creating or switching target sessions now stay attached to the intended workspace directory.
+- Files: chat and tool links now handle Windows drive-letter and backslash paths.