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/ui/styles_test.go

@@ -92,9 +92,10 @@ func TestRenderTypeVariants(t *testing.T) {
 		{"task", TypeTaskStyle.Render("task")},
 		{"epic", TypeEpicStyle.Render("epic")},
 		{"chore", TypeChoreStyle.Render("chore")},
-		{"agent", TypeAgentStyle.Render("agent")},
-		{"role", TypeRoleStyle.Render("role")},
-		{"rig", TypeRigStyle.Render("rig")},
+		// Gas Town types (agent, role, rig) have been removed - they now fall through to default
+		{"agent", "agent"},  // Falls through to default (no styling)
+		{"role", "role"},    // Falls through to default (no styling)
+		{"rig", "rig"},      // Falls through to default (no styling)
 		{"custom", "custom"},
 	}
 	for _, tc := range cases {