refactor(types): remove Gas Town type constants from beads core (bd-w2zz4)

Remove Gas Town-specific type constants (TypeMolecule, TypeGate, TypeConvoy,
TypeMergeRequest, TypeSlot, TypeAgent, TypeRole, TypeRig, TypeEvent, TypeMessage)
from internal/types/types.go.

Beads now only has core work types built-in:
- bug, feature, task, epic, chore

All Gas Town types are now purely custom types with no special handling in beads.
Use string literals like "gate" or "molecule" when needed, and configure
types.custom in config.yaml for validation.

Changes:
- Remove Gas Town type constants from types.go
- Remove mr/mol aliases from Normalize()
- Update bd types command to only show core types
- Replace all constant usages with string literals throughout codebase
- Update tests to use string literals

This decouples beads from Gas Town, making it a generic issue tracker.

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

cmd/bd/cli_coverage_show_test.go

@@ -391,9 +391,9 @@ func TestCoverage_ShowThread(t *testing.T) {
 	}
 	ctx := context.Background()
 
-	root := &types.Issue{Title: "Root message", IssueType: types.TypeMessage, Status: types.StatusOpen, Sender: "alice", Assignee: "bob"}
-	reply1 := &types.Issue{Title: "Re: Root", IssueType: types.TypeMessage, Status: types.StatusOpen, Sender: "bob", Assignee: "alice"}
-	reply2 := &types.Issue{Title: "Re: Re: Root", IssueType: types.TypeMessage, Status: types.StatusOpen, Sender: "alice", Assignee: "bob"}
+	root := &types.Issue{Title: "Root message", IssueType: "message", Status: types.StatusOpen, Sender: "alice", Assignee: "bob"}
+	reply1 := &types.Issue{Title: "Re: Root", IssueType: "message", Status: types.StatusOpen, Sender: "bob", Assignee: "alice"}
+	reply2 := &types.Issue{Title: "Re: Re: Root", IssueType: "message", Status: types.StatusOpen, Sender: "alice", Assignee: "bob"}
 	if err := s.CreateIssue(ctx, root, "test-user"); err != nil {
 		s.Close()
 		t.Fatalf("CreateIssue root: %v", err)