feat(tui): render Mermaid GitGraph diagrams

[kitlangton]

What

Render Mermaid gitGraph fences as terminal-native vertical commit graphs in OpenCode.

○    baseline
│╲
│ ○  extract-seam Extract seam
│ │
│ ○  add-tests  (refactor, tag: ready)
│ │
○ │  unrelated-fix
│╱
◎    land-refactor  (main, tag: v2)

The vertical layout follows terminal Git graph conventions: branches staircase outward, remain vertical while active, and collapse directly into merge nodes without dangling corner glyphs.

How

• Adds parsing for commits, branches, checkout/switch, merges, IDs, messages, tags, commit types, branch ordering, comments, titles, and accessibility metadata.
• Preserves final branch heads and validates invalid branch and merge operations with source diagnostics.
• Renders semantic commit types and colored branch lanes with auxiliary rows for wide transitions.
• Integrates detection, themed Markdown rendering, streaming fallback, and tests.
• Keeps the implementation synchronized byte-for-byte with the stacked @opentui/mermaid implementation.

Scope

• This PR is stacked on feat(tui): render Mermaid timelines #42130 and contains only GitGraph support beyond that branch.
• Mermaid orientations are preserved while terminal output intentionally uses a vertical layout.
• cherry-pick is explicitly rejected for now.

Testing

• bun run test in packages/merman: 324 passed
• bun run typecheck in packages/merman
• Full 39-package push-hook typecheck
• GitGraph family is byte-for-byte synchronized with OpenTUI