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/export_obsidian.go

@@ -33,6 +33,8 @@ var obsidianPriority = []string{
 }
 
 // obsidianTypeTag maps bd issue type to Obsidian tag
+// Note: Gas Town-specific types (agent, role, rig, convoy, slot) are now labels.
+// The labels will be converted to tags automatically via the label->tag logic.
 var obsidianTypeTag = map[types.IssueType]string{
 	types.TypeBug:          "#Bug",
 	types.TypeFeature:      "#Feature",
@@ -43,10 +45,6 @@ var obsidianTypeTag = map[types.IssueType]string{
 	types.TypeMergeRequest: "#MergeRequest",
 	types.TypeMolecule:     "#Molecule",
 	types.TypeGate:         "#Gate",
-	types.TypeAgent:        "#Agent",
-	types.TypeRole:         "#Role",
-	types.TypeRig:          "#Rig",
-	types.TypeConvoy:       "#Convoy",
 	types.TypeEvent:        "#Event",
 }