feat(#2163): opt-in git branch/status segment in the statusline

New statusline.show_git config (default false). When enabled, a git
segment renders after the directory: current branch plus compact
work-state markers (+staged ~unstaged ?untracked ↑ahead ↓behind, or ✓
when clean and in sync), e.g. " │ main+2~1?3".

One git status --porcelain=v2 --branch spawn per render via execFileSync
with a fixed argument array (no shell), a 1.5s timeout, and the
workspace dir passed with -C. Fails silently — segment absent outside a
repo, without git, or on timeout. Default output is unchanged when the
flag is absent.

Claude-Session: https://claude.ai/code/session_01Hme55Pvq6BhpgwBcyC5HAg

src/config.cts

@@ -770,6 +770,13 @@ function cmdConfigSet(cwd: string, keyPath: string | undefined, value: string |
     }
   }
 
+  // statusline.show_git — boolean only
+  if (kp === 'statusline.show_git') {
+    if (typeof parsedValue !== 'boolean') {
+      error(`Invalid statusline.show_git '${val}'. Must be a boolean (true or false).`);
+    }
+  }
+
   // Fallow scope + profile enum validation (#3424)
   const VALID_FALLOW_SCOPES = ['phase', 'repo'];
   if (kp === 'code_quality.fallow.scope') assertEnumValue(parsedValue, val, VALID_FALLOW_SCOPES, 'code_quality.fallow.scope');