feat(statusline): show update indicator when new GSD version available

Adds SessionStart hook that checks npm in background, caches result.
Statusline shows "⬆ /gsd:update │" when update available.

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

commands/gsd/update.md

@@ -85,6 +85,12 @@ npx get-shit-done-cc --global
 ```
 
 Capture output. If install fails, show error and STOP.
+
+Clear the update cache so statusline indicator disappears:
+
+```bash
+rm -f ~/.claude/cache/gsd-update-check.json
+```
 </step>
 
 <step name="fetch_changelog">