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/show_unit_helpers_test.go

@@ -100,6 +100,10 @@ func TestFindRepliesToAndReplies_WorksWithMemoryStorage(t *testing.T) {
 	if err := st.SetConfig(ctx, "issue_prefix", "test"); err != nil {
 		t.Fatalf("SetConfig: %v", err)
 	}
+	// Configure Gas Town custom types for test compatibility (bd-find4)
+	if err := st.SetConfig(ctx, "types.custom", "message"); err != nil {
+		t.Fatalf("SetConfig types.custom: %v", err)
+	}
 
 	root := &types.Issue{Title: "root", Status: types.StatusOpen, Priority: 2, IssueType: types.TypeMessage, Sender: "a", Assignee: "b"}
 	reply1 := &types.Issue{Title: "r1", Status: types.StatusOpen, Priority: 2, IssueType: types.TypeMessage, Sender: "b", Assignee: "a"}