feat(doctor): Add workspace-level health checks (gt-f9x.5)

Implements 6 workspace doctor checks:
- TownConfigExists: Verify mayor/town.json exists
- TownConfigValid: Validate town.json has required fields (type, version, name)
- RigsRegistryExists: Check mayor/rigs.json exists (fixable: creates empty)
- RigsRegistryValid: Verify registered rigs exist on disk (fixable: removes missing)
- MayorExists: Check mayor/ directory structure
- MayorStateValid: Validate mayor/state.json JSON (fixable: resets to default)

Added WorkspaceChecks() helper to return all workspace checks for registration.

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

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

internal/cmd/doctor.go

@@ -24,6 +24,14 @@ var doctorCmd = &cobra.Command{
 Doctor checks for common configuration issues, missing files,
 and other problems that could affect workspace operation.
 
+Workspace checks:
+  - town-config-exists       Check mayor/town.json exists
+  - town-config-valid        Check mayor/town.json is valid
+  - rigs-registry-exists     Check mayor/rigs.json exists (fixable)
+  - rigs-registry-valid      Check registered rigs exist (fixable)
+  - mayor-exists             Check mayor/ directory structure
+  - mayor-state-valid        Check mayor/state.json is valid (fixable)
+
 Infrastructure checks:
   - daemon                   Check if daemon is running (fixable)
   - boot-health              Check Boot watchdog health (vet mode)
@@ -85,6 +93,9 @@ func runDoctor(cmd *cobra.Command, args []string) error {
 	// Create doctor and register checks
 	d := doctor.NewDoctor()
 
+	// Register workspace-level checks first (fundamental)
+	d.RegisterAll(doctor.WorkspaceChecks()...)
+
 	// Register built-in checks
 	d.Register(doctor.NewTownGitCheck())
 	d.Register(doctor.NewDaemonCheck())