feat: add bd doctor --check-health for lightweight git hook health checks

- Add --check-health flag for quick, silent health checks (exit 0 on success)
- Check version mismatch (CLI vs database), sync.branch config, outdated hooks
- Add hints.doctor config option to suppress doctor hints globally
- Update post-merge/post-checkout hooks to call bd doctor --check-health
- Suggest running bd doctor in upgrade notification
- Modernize post-checkout hook (bash→sh, use bd sync instead of bd import)

🤖 Generated with [Claude Code](https://claude.com/claude-code)

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

cmd/bd/version_tracking.go

@@ -122,6 +122,7 @@ func maybeShowUpgradeNotification() {
 	// Display notification
 	fmt.Printf("🔄 bd upgraded from v%s to v%s since last use\n", previousVersion, Version)
 	fmt.Println("💡 Run 'bd upgrade review' to see what changed")
+	fmt.Println("💊 Run 'bd doctor' to verify upgrade completed cleanly")
 
 	// Check if BD_GUIDE.md exists and needs updating
 	checkAndSuggestBDGuideUpdate()