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

package.json

@@ -42,6 +42,8 @@
     "test": "node scripts/run-isolated-tests.mjs scripts && bun run --cwd packages/ui test && bun run --cwd packages/vscode test && bun run --cwd packages/electron test && bun run --cwd packages/web test",
     "clean": "bun run --filter '*' clean",
     "changelog-card": "node scripts/changelog-card/generate.mjs",
+    "changelog:build": "node scripts/changelog/build.mjs",
+    "changelog:check": "node scripts/changelog/build.mjs --check",
     "postinstall": "node ./fix-deprecation.js && patch-package && node ./packages/electron/scripts/ensure-electron.mjs --best-effort",
     "dev:web": "bun run --cwd packages/web build:watch",
     "dev:web:server": "bun run --cwd packages/web dev:server:watch",
@@ -81,7 +83,7 @@
     "icons:generate": "bun run scripts/generate-icon-sprite.mjs",
     "themes:port:opencode": "tsx scripts/port-opencode-theme.ts",
     "version:bump": "node scripts/bump-version.mjs",
-    "release:prepare": "bun run build && bun run type-check && bun run lint",
+    "release:prepare": "bun run changelog:check && bun run build && bun run type-check && bun run lint",
     "release:test": "./scripts/test-release-build.sh",
     "release:test:intel": "./scripts/test-release-build.sh x86_64",
     "release:test:arm": "./scripts/test-release-build.sh aarch64",