Add role-based theming with layered config and doctor check

Role themes:
- witness: rust (red/alert)
- refinery: plum (purple)
- crew/polecat: inherit rig theme

Resolution order:
1. Per-rig role override (rig/.gastown/config.json role_themes)
2. Global role default (mayor/town.json theme.role_defaults)
3. Built-in role defaults
4. Rig theme (config or hash-based)

Config schema:
- TownConfig.Theme.RoleDefaults: global role->theme map
- RigConfig.Theme.RoleThemes: per-rig role overrides

Doctor check:
- Detects sessions with outdated theme format (brackets)
- Fixable with 'gt theme apply --all'

🤖 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 +61,7 @@ func runDoctor(cmd *cobra.Command, args []string) error {
 	d.Register(doctor.NewBranchCheck())
 	d.Register(doctor.NewBeadsSyncOrphanCheck())
 	d.Register(doctor.NewIdentityCollisionCheck())
+	d.Register(doctor.NewThemeCheck())
 
 	// Ephemeral beads checks
 	d.Register(doctor.NewEphemeralExistsCheck())