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/create.go

@@ -474,7 +474,15 @@ var createCmd = &cobra.Command{
 		}
 
 		// Auto-add role_type/rig labels for agent beads (enables filtering queries)
-		if issue.IssueType == types.TypeAgent {
+		// Check for gt:agent label to identify agent beads (Gas Town separation)
+		hasAgentLabel := false
+		for _, l := range labels {
+			if l == "gt:agent" {
+				hasAgentLabel = true
+				break
+			}
+		}
+		if hasAgentLabel {
 			if issue.RoleType != "" {
 				agentLabel := "role_type:" + issue.RoleType
 				if err := store.AddLabel(ctx, issue.ID, agentLabel, actor); err != nil {