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/rpc/rpc_test.go

@@ -51,6 +51,13 @@ func setupTestServer(t *testing.T) (*Server, *Client, func()) {
 		t.Fatalf("Failed to set issue_prefix: %v", err)
 	}
 
+	// Configure Gas Town custom types for test compatibility (bd-find4)
+	if err := store.SetConfig(ctx, "types.custom", "molecule,gate,convoy,merge-request,slot,agent,role,rig,event,message"); err != nil {
+		store.Close()
+		os.RemoveAll(tmpDir)
+		t.Fatalf("Failed to set types.custom: %v", err)
+	}
+
 	server := NewServer(socketPath, store, tmpDir, dbPath)
 
 	ctx, cancel := context.WithCancel(context.Background())