feat(tui): display git branch in sidebar and landing page

[processtrader]

Summary

• Display the current git branch name in the sidebar (below working directory) and on the landing page
• Live refresh via fsnotify watcher on .git/HEAD and refs/heads/ — branch updates instantly on git checkout, git switch, rebase, merge, etc.
• Supports worktrees by resolving .git file to the actual git directory
• Gracefully hides when not in a git repo or in detached HEAD state

Implementation details

• Async tea.Cmd pattern — never blocks render
• Cached in UI.gitBranch, only re-runs git branch --show-current when .git/HEAD or refs/heads/ changes on disk
• New GitBranchIcon constant and Sidebar.GitBranch style following existing conventions
• MaxHeight(1) prevents long branch names from wrapping in the narrow sidebar

Test plan

• TestLoadGitBranch/returns_empty_for_non_git_dir — no crash, empty result, no watcher
• TestLoadGitBranch/returns_branch_for_git_repo_and_starts_watcher — correct branch, watcher started
• TestLoadGitBranch/returns_empty_for_detached_head — graceful fallback
• TestLoadGitBranch/watchGitBranch_returns_nil_when_no_watcher — nil safety
• Full model test suite passes
• Manual: run go run . in a git repo, verify branch shows in sidebar
• Manual: run git checkout -b test-branch in another terminal, verify sidebar updates live
• Manual: verify no branch shown in a non-git directory

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