Display current git branch in session UI

Feature request: Display the current git branch in the session header or prompt.

Use case: When working with multiple Claude Code sessions across different git worktrees/branches, it's difficult to track which session is working on which branch. This is especially challenging when using worktrees for parallel development.

Proposed solution: Display the current git branch (from git branch --show-current) in:

• Session title/header
• Command prompt
• Or as a persistent status indicator

Current workaround: Claude can run git branch --show-current at the start of every response