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

cmd/bd/merge_slot.go

@@ -25,7 +25,7 @@ A merge slot is an exclusive access primitive: only one agent can hold it at a t
 This prevents "monkey knife fights" where multiple polecats race to resolve conflicts
 and create cascading conflicts.
 
-Each rig has one merge slot bead: <prefix>-merge-slot (type=slot).
+Each rig has one merge slot bead: <prefix>-merge-slot (labeled gt:slot).
 The slot uses:
   - status=open: slot is available
   - status=in_progress: slot is held
@@ -157,15 +157,15 @@ func runMergeSlotCreate(cmd *cobra.Command, args []string) error {
 	// Create the merge slot bead
 	title := "Merge Slot"
 	description := "Exclusive access slot for serialized conflict resolution in the merge queue."
-	slotType := types.TypeSlot
 
 	if daemonClient != nil {
 		createArgs := &rpc.CreateArgs{
 			ID:          slotID,
 			Title:       title,
 			Description: description,
-			IssueType:   string(slotType),
-			Priority:    0, // P0 - system infrastructure
+			IssueType:   string(types.TypeTask), // Use task type; gt:slot label marks it as slot
+			Priority:    0,                      // P0 - system infrastructure
+			Labels:      []string{"gt:slot"},   // Gas Town slot label
 		}
 		resp, err := daemonClient.Create(createArgs)
 		if err != nil {
@@ -179,13 +179,18 @@ func runMergeSlotCreate(cmd *cobra.Command, args []string) error {
 			ID:          slotID,
 			Title:       title,
 			Description: description,
-			IssueType:   slotType,
+			IssueType:   types.TypeTask, // Use task type; gt:slot label marks it as slot
 			Status:      types.StatusOpen,
 			Priority:    0,
 		}
 		if err := store.CreateIssue(ctx, issue, actor); err != nil {
 			return fmt.Errorf("failed to create merge slot: %w", err)
 		}
+		// Add gt:slot label to mark as slot bead
+		if err := store.AddLabel(ctx, slotID, "gt:slot", actor); err != nil {
+			// Non-fatal: log warning but don't fail creation
+			fmt.Fprintf(os.Stderr, "warning: failed to add gt:slot label: %v\n", err)
+		}
 		markDirtyAndScheduleFlush()
 	}