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/thread_test.go

@@ -24,7 +24,7 @@ func TestThreadTraversal(t *testing.T) {
 		Description: "This is the original message",
 		Status:      types.StatusOpen,
 		Priority:    2,
-		IssueType:   types.TypeMessage,
+		IssueType:   "message",
 		Assignee:    "worker",
 		Sender:      "manager",
 		Ephemeral:        true,
@@ -40,7 +40,7 @@ func TestThreadTraversal(t *testing.T) {
 		Description: "This is reply 1",
 		Status:      types.StatusOpen,
 		Priority:    2,
-		IssueType:   types.TypeMessage,
+		IssueType:   "message",
 		Assignee:    "manager",
 		Sender:      "worker",
 		Ephemeral:        true,
@@ -56,7 +56,7 @@ func TestThreadTraversal(t *testing.T) {
 		Description: "This is reply 2",
 		Status:      types.StatusOpen,
 		Priority:    2,
-		IssueType:   types.TypeMessage,
+		IssueType:   "message",
 		Assignee:    "worker",
 		Sender:      "manager",
 		Ephemeral:        true,
@@ -187,7 +187,7 @@ func TestThreadTraversalEmptyThread(t *testing.T) {
 		Description: "This message has no thread",
 		Status:      types.StatusOpen,
 		Priority:    2,
-		IssueType:   types.TypeMessage,
+		IssueType:   "message",
 		Assignee:    "user",
 		Sender:      "sender",
 		Ephemeral:        true,
@@ -225,7 +225,7 @@ func TestThreadTraversalBranching(t *testing.T) {
 		Description: "This message will have multiple replies",
 		Status:      types.StatusOpen,
 		Priority:    2,
-		IssueType:   types.TypeMessage,
+		IssueType:   "message",
 		Assignee:    "user",
 		Sender:      "sender",
 		Ephemeral:        true,
@@ -242,7 +242,7 @@ func TestThreadTraversalBranching(t *testing.T) {
 		Description: "First branch reply",
 		Status:      types.StatusOpen,
 		Priority:    2,
-		IssueType:   types.TypeMessage,
+		IssueType:   "message",
 		Assignee:    "sender",
 		Sender:      "user",
 		Ephemeral:        true,
@@ -258,7 +258,7 @@ func TestThreadTraversalBranching(t *testing.T) {
 		Description: "Second branch reply",
 		Status:      types.StatusOpen,
 		Priority:    2,
-		IssueType:   types.TypeMessage,
+		IssueType:   "message",
 		Assignee:    "sender",
 		Sender:      "another-user",
 		Ephemeral:        true,
@@ -361,7 +361,7 @@ func TestThreadTraversalOnlyRepliesTo(t *testing.T) {
 		Description: "First message (target of blocks dep)",
 		Status:      types.StatusOpen,
 		Priority:    2,
-		IssueType:   types.TypeMessage,
+		IssueType:   "message",
 		Assignee:    "user",
 		Sender:      "sender",
 		Ephemeral:        true,
@@ -377,7 +377,7 @@ func TestThreadTraversalOnlyRepliesTo(t *testing.T) {
 		Description: "Second message with blocks dependency to msg1",
 		Status:      types.StatusOpen,
 		Priority:    2,
-		IssueType:   types.TypeMessage,
+		IssueType:   "message",
 		Assignee:    "user",
 		Sender:      "sender",
 		Ephemeral:        true,