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

gsd-core/bin/shared/config-schema.manifest.json

@@ -72,6 +72,7 @@
     "statusline.show_last_command",
     "statusline.context_position",
     "statusline.show_context_tokens",
+    "statusline.show_git",
     "workflow.max_discuss_passes",
     "features.thinking_partner",
     "context",