feat(tui): append git branch to cwd label in status bar

Adds useGitBranch hook (async, cached, 15s TTL) and fmtCwdBranch
helper so the footer shows `~/repo (main)` instead of just `~/repo`.
Degrades silently when git is unavailable or cwd is outside a repo.

Partial fix for #12267 (TUI portion; #12277 covers the Python side).

ui-tui/src/domain/paths.ts

@@ -4,3 +4,14 @@ export const shortCwd = (cwd: string, max = 28) => {
 
   return p.length <= max ? p : `…${p.slice(-(max - 1))}`
 }
+
+export const fmtCwdBranch = (cwd: string, branch: null | string, max = 40) => {
+  if (!branch) {
+    return shortCwd(cwd, max)
+  }
+
+  const b = branch.length > 16 ? `…${branch.slice(-15)}` : branch
+  const tag = ` (${b})`
+
+  return `${shortCwd(cwd, Math.max(8, max - tag.length))}${tag}`
+}