feat(doctor): add ephemeral beads health checks

Add gt doctor checks for ephemeral beads repos:
- ephemeral-exists: Verify .beads-ephemeral/ exists for each rig (fixable)
- ephemeral-git: Verify it's a valid git repo (fixable)
- ephemeral-orphans: Detect molecules >24h old (needs manual review)
- ephemeral-size: Warn if repo >100MB
- ephemeral-stale: Detect molecules with no activity in last hour

Implements gt-3x0z.3.

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

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

internal/cmd/doctor.go

@@ -61,6 +61,13 @@ func runDoctor(cmd *cobra.Command, args []string) error {
 	d.Register(doctor.NewBranchCheck())
 	d.Register(doctor.NewBeadsSyncOrphanCheck())
 
+	// Ephemeral beads checks
+	d.Register(doctor.NewEphemeralExistsCheck())
+	d.Register(doctor.NewEphemeralGitCheck())
+	d.Register(doctor.NewEphemeralOrphansCheck())
+	d.Register(doctor.NewEphemeralSizeCheck())
+	d.Register(doctor.NewEphemeralStaleCheck())
+
 	// Run checks
 	var report *doctor.Report
 	if doctorFix {