feat(types): remove Gas Town types from core built-in types

Core beads built-in types now only include work types:
- bug, feature, task, epic, chore

Gas Town types (molecule, gate, convoy, merge-request, slot, agent,
role, rig, event, message) are now "well-known custom types":
- Constants still exist for code convenience
- Require types.custom configuration for validation
- bd types command shows core types and configured custom types

Changes:
- types.go: Separate core work types from well-known custom types
- IsValid(): Only accepts core work types
- bd types: Updated to show core types and custom types from config
- memory.go: Use ValidateWithCustom for custom type support
- multirepo.go: Only check core types as built-in
- Updated all tests to configure custom types

This allows Gas Town (and other projects) to define their own types
via config while keeping beads core focused on work tracking.

Closes: bd-find4

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

cmd/bd/doctor/multirepo.go

@@ -209,10 +209,10 @@ func findUnknownTypesInHydratedIssues(repoPath string, multiRepo *config.MultiRe
 	}
 	defer db.Close()
 
-	// Collect all known types (built-in + parent custom + all child custom)
+	// Collect all known types (core work types + parent custom + all child custom)
+	// Only core work types are built-in; Gas Town types require types.custom config.
 	knownTypes := map[string]bool{
 		"bug": true, "feature": true, "task": true, "epic": true, "chore": true,
-		"message": true, "merge-request": true, "molecule": true, "gate": true, "event": true,
 	}
 
 	// Add parent's custom types