feat(tui): show session branches in vertical tabs

[kitlangton]

What

Show each vertical session tab's non-default VCS branch on its existing project detail row.

• default branches remain hidden to keep ordinary sessions quiet
• feature branches follow the project name as project:branch in the same subdued metadata color
• long metadata rows fade at the right edge like session titles, without an ellipsis
• sessions resolve VCS state from their own directory/workspace rather than the currently focused repository

How

• session-tabs.tsx composes project and branch into the existing subdued detail row and applies its terminal fade.
• session-tabs-model.ts defines the default-branch visibility rule.
• context/session-tabs.tsx extends the existing lightweight open-tab warmup to fetch location-scoped VCS metadata after session metadata resolves.
• Existing vcs.branch.updated handling keeps the displayed branch current after checkout.

Scope

This only changes vertical tabs. Horizontal tabs do not have a second metadata row. The treatment intentionally avoids font-specific branch icons and additional semantic colors.

Testing

• bun run test in packages/tui: 610 passed, 5 skipped
• bun run typecheck in packages/tui
• workspace bun run typecheck via the push hook: 33 packages passed
• focused coverage verifies default-branch suppression and VCS hydration across distinct session locations
• OpenTUI capture of default, feature, and long branch labels in the vertical 12-tab fixture

Demo

The first row is on its default branch and shows only the project. The remaining rows use compact project:branch metadata; the third row demonstrates the right-edge fade for a long branch.