feat(doctor): Add repo-fingerprint check for beads database (gt-nrgm5)

Add repo-fingerprint check to gt doctor that verifies beads databases have
valid repository fingerprints. Missing or empty fingerprints can cause daemon
startup failures and sync issues.

The check:
- Uses bd doctor --json to check fingerprint status
- Runs on town-level and rig-level beads directories
- Can fix by running bd migrate --update-repo-id
- Restarts daemon after migration if it was running

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

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

internal/cmd/doctor.go

@@ -34,6 +34,7 @@ Workspace checks:
 
 Infrastructure checks:
   - daemon                   Check if daemon is running (fixable)
+  - repo-fingerprint         Check database has valid repo fingerprint (fixable)
   - boot-health              Check Boot watchdog health (vet mode)
 
 Cleanup checks (fixable):
@@ -99,6 +100,7 @@ func runDoctor(cmd *cobra.Command, args []string) error {
 	// Register built-in checks
 	d.Register(doctor.NewTownGitCheck())
 	d.Register(doctor.NewDaemonCheck())
+	d.Register(doctor.NewRepoFingerprintCheck())
 	d.Register(doctor.NewBootHealthCheck())
 	d.Register(doctor.NewBeadsDatabaseCheck())
 	d.Register(doctor.NewBdDaemonCheck())