feat: extract Gas Town types from beads core (bd-i54l)

Remove Gas Town-specific issue types (agent, role, rig, convoy, slot)
from beads core. These types are now identified by labels instead:
- gt:agent, gt:role, gt:rig, gt:convoy, gt:slot

Changes:
- internal/types/types.go: Remove TypeAgent, TypeRole, TypeRig, TypeConvoy, TypeSlot constants
- cmd/bd/agent.go: Create agents with TypeTask + gt:agent label
- cmd/bd/merge_slot.go: Create slots with TypeTask + gt:slot label
- internal/storage/sqlite/queries.go, transaction.go: Query convoys by gt:convoy label
- internal/rpc/server_issues_epics.go: Check gt:agent label for role_type/rig label auto-add
- cmd/bd/create.go: Check gt:agent label for role_type/rig label auto-add
- internal/ui/styles.go: Remove agent/role/rig type colors
- cmd/bd/export_obsidian.go: Remove agent/role/rig/convoy type tag mappings
- Update all affected tests

This enables beads to be a generic issue tracker while Gas Town
uses labels for its specific type semantics.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

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

Executed-By: beads/crew/dave
Rig: beads
Role: crew

internal/storage/sqlite/sqlite_test.go

@@ -1475,17 +1475,20 @@ func TestConvoyReactiveCompletion(t *testing.T) {
 
 	ctx := context.Background()
 
-	// Create a convoy
+	// Create a convoy (using task type with gt:convoy label)
 	convoy := &types.Issue{
 		Title:     "Test Convoy",
 		Status:    types.StatusOpen,
 		Priority:  2,
-		IssueType: types.TypeConvoy,
+		IssueType: types.TypeTask, // Use task type; gt:convoy label marks it as convoy
 	}
 	err := store.CreateIssue(ctx, convoy, "test-user")
 	if err != nil {
 		t.Fatalf("CreateIssue convoy failed: %v", err)
 	}
+	if err := store.AddLabel(ctx, convoy.ID, "gt:convoy", "test-user"); err != nil {
+		t.Fatalf("Failed to add gt:convoy label: %v", err)
+	}
 
 	// Create two issues to track
 	issue1 := &types.Issue{