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/validation/bead_test.go

@@ -126,10 +126,10 @@ func TestParseIssueType(t *testing.T) {
 		{"merge-request type", "merge-request", types.TypeMergeRequest, false, ""},
 		{"molecule type", "molecule", types.TypeMolecule, false, ""},
 		{"gate type", "gate", types.TypeGate, false, ""},
-		{"agent type", "agent", types.TypeAgent, false, ""},
-		{"role type", "role", types.TypeRole, false, ""},
-		{"rig type", "rig", types.TypeRig, false, ""},
+		{"event type", "event", types.TypeEvent, false, ""},
 		{"message type", "message", types.TypeMessage, false, ""},
+		// Gas Town types (agent, role, rig, convoy, slot) have been removed
+		// They now require custom type configuration,
 
 		// Case sensitivity (function is case-sensitive)
 		{"uppercase bug", "BUG", types.TypeTask, true, "invalid issue type"},