feat(tui): render Mermaid timelines

[kitlangton]

What

Render Mermaid timeline fences as terminal-native vertical timelines in OpenCode instead of falling back to source code.

       Product history

Foundation ───┐
              │
      2024 ───●  Prototype
              │  First release

Bare, TD, and LR timeline headers preserve Mermaid parse semantics while sharing the vertical layout that remains readable in narrow terminal viewports.

How

• Adds timeline detection, parsing, vertical-spine drawing, styling, and plain rendering to packages/merman/src/timeline.
• Supports titles, sections, standalone periods, inline and continued events, entities, <br> lines, comments, and accessibility metadata.
• Integrates timeline output with the existing themed Markdown renderer and streaming fallback behavior.
• Keeps the implementation synchronized with @opentui/mermaid in feat(mermaid): add terminal diagram renderer opentui#1351 until OpenCode can consume that package directly.

Scope

This intentionally renders timeline LR vertically. Horizontal timelines become unreadable quickly in terminal message viewports. Mermaid-specific timeline styling and icons remain unsupported.

Testing

• bun run test in packages/merman: 311 passed
• bun run typecheck in packages/merman
• Push hook: full monorepo typecheck, 32 packages passed