feat(doctor): Add bd daemon health check with actionable error messages

Add a new doctor check that verifies the bd (beads) daemon is running
and healthy. When the daemon fails to start, the check:

- Surfaces specific error messages (legacy database, repo mismatch)
- Provides one-liner fix commands
- Auto-fixes by running bd migrate --update-repo-id when appropriate

This addresses GH #25: gt status slow when bd daemon not running.

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

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

internal/cmd/doctor.go

@@ -101,6 +101,7 @@ func runDoctor(cmd *cobra.Command, args []string) error {
 	d.Register(doctor.NewDaemonCheck())
 	d.Register(doctor.NewBootHealthCheck())
 	d.Register(doctor.NewBeadsDatabaseCheck())
+	d.Register(doctor.NewBdDaemonCheck())
 	d.Register(doctor.NewPrefixConflictCheck())
 	d.Register(doctor.NewRoutesCheck())
 	d.Register(doctor.NewOrphanSessionCheck())