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/1.7.4.md

@@ -0,0 +1,43 @@
+---
+version: 1.7.4
+date: 2026-02-24
+---
+
+## App
+
+### New
+- Chat: added fullscreen Mermaid preview, persisted default thinking variant selection, and hardened file-preview safety checks for a safer, more predictable message experience (thanks to @yulia-ivashko).
+- Settings: added an MCP config manager UI to simplify editing and validating MCP server configuration (thanks to @nguyenngothuong).
+- Files: added C, C++, and Go language support for syntax-aware rendering in code-heavy workflows (thanks to @fomenks).
+
+### Improvements
+- Settings: redesigned the settings workspace with flatter, more consistent page layouts.
+- Settings: improved agents and skills navigation by grouping entries by subfolder for easier management at scale (thanks to @nguyenngothuong).
+- Chat: draft text now persists per session, and the input supports an expanded focus mode for longer prompts (thanks to @nguyenngothuong).
+- Sessions: expanded folder management with subfolders, cleaner organization actions, and clearer delete confirmations (thanks to @nguyenngothuong).
+- Git/PR: moved commit-message and PR-description generation to active-session structured output.
+- Chat Activity: improved Structured Output tool rendering with dedicated title/icon, clearer result descriptions, and more reliable detailed expansion defaults.
+- Notifications/Voice: moved utility model controls into AI Summarization as a Zen-only Summarization Model setting.
+- Mobile: refreshed drawer and session-status layouts (thanks to @Jovines).
+- Desktop: improved remote instance URL handling (thanks to @shekohex).
+
+### Fixes
+- Chat: improved streaming smoothness and stability with buffered updates and runtime fixes.
+
+## VS Code
+
+### New
+- Chat: added fullscreen Mermaid preview, improved default thinking-variant persistence, and hardened file-preview safety checks for a more predictable message experience (thanks to @yulia-ivashko).
+- Settings: added an MCP config manager UI to simplify editing and validating MCP server configuration (thanks to @nguyenngothuong).
+- Chat: added C, C++, and Go language support for syntax-aware rendering in code-heavy workflows (thanks to @fomenks).
+
+### Improvements
+- Settings: redesigned the settings workspace with flatter, more consistent layouts.
+- Settings: grouped agents/skills navigation by subfolder to make larger setups easier to manage (thanks to @nguyenngothuong).
+- Chat: draft text now persists per session, and the input supports an expanded focus mode for longer prompts (thanks to @nguyenngothuong).
+- Sessions: expanded folder management with subfolders, cleaner organization controls, and clearer delete confirmations (thanks to @nguyenngothuong).
+- Chat Activity: improved Structured Output tool rendering with dedicated title/icon, clearer result descriptions, and more reliable detailed expansion defaults.
+- Reliability: aligned file read/raw endpoint safety checks with other runtimes (thanks to @yulia-ivashko).
+
+### Fixes
+- Chat: improved streaming smoothness and runtime stability with buffered updates and reliability fixes.