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>

internal/molecules/molecules_test.go

@@ -81,6 +81,11 @@ func TestLoader_LoadAll(t *testing.T) {
 		t.Fatalf("Failed to set prefix: %v", err)
 	}
 
+	// Configure custom types for Gas Town types (bd-find4)
+	if err := store.SetConfig(ctx, "types.custom", "molecule"); err != nil {
+		t.Fatalf("Failed to set types.custom: %v", err)
+	}
+
 	// Create a project-level molecules.jsonl
 	moleculesPath := filepath.Join(beadsDir, "molecules.jsonl")
 	content := `{"id":"mol-feature","title":"Feature Template","issue_type":"molecule","status":"open","description":"Standard feature workflow"}
@@ -150,6 +155,11 @@ func TestLoader_SkipExistingMolecules(t *testing.T) {
 		t.Fatalf("Failed to set prefix: %v", err)
 	}
 
+	// Configure custom types for Gas Town types (bd-find4)
+	if err := store.SetConfig(ctx, "types.custom", "molecule"); err != nil {
+		t.Fatalf("Failed to set types.custom: %v", err)
+	}
+
 	// Pre-create a molecule in the database (skip prefix validation for mol-* IDs)
 	existingMol := &types.Issue{
 		ID:         "mol-existing",