[LawrenceYoong]

What changed

• Keep the Docs shortcut visible in the Open menu from the office map, so it opens the current project's docs without walking to the bookshelf.
• Add a Mermaid system overview and task flow as editable Markdown, and document the shortcut and diagram.
• Render Mermaid diagrams in the bookshelf.

Verification

• npm run typecheck passed.
• npm run build passed.
• npm test: 613 passed, 1 failed in the existing multi-repository PR test (tests/repos.test.ts): expected web.base, got undefined; reproduced when run alone.
• Headless screenshot could not run: Chromium requires libnspr4.so, unavailable in the environment; installing system dependencies needs sudo.