refactor: remove bd mail commands - mail moves to Gas Town

Mail is orchestration, not data plane. The bd mail commands were just
sugar over bd create/list/show with type=message. Gas Town (gt) now
owns mail routing and delivery.

Removed:
- cmd/bd/mail.go - all mail subcommands (send, inbox, read, ack, reply)
- cmd/bd/mail_test.go - mail command tests
- docs/messaging.md - dedicated mail documentation
- EventMessage hook - no longer triggered

Kept (data plane):
- type=message as valid issue type
- Sender, Ephemeral fields on issues
- replies_to dependency type for threading

Coordination: gt-9xg implements native mail in Gas Town.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

internal/hooks/hooks.go

@@ -12,18 +12,16 @@ import (
 
 // Event types
 const (
-	EventCreate  = "create"
-	EventUpdate  = "update"
-	EventClose   = "close"
-	EventMessage = "message"
+	EventCreate = "create"
+	EventUpdate = "update"
+	EventClose  = "close"
 )
 
 // Hook file names
 const (
-	HookOnCreate  = "on_create"
-	HookOnUpdate  = "on_update"
-	HookOnClose   = "on_close"
-	HookOnMessage = "on_message"
+	HookOnCreate = "on_create"
+	HookOnUpdate = "on_update"
+	HookOnClose  = "on_close"
 )
 
 // Runner handles hook execution
@@ -120,8 +118,6 @@ func eventToHook(event string) string {
 		return HookOnUpdate
 	case EventClose:
 		return HookOnClose
-	case EventMessage:
-		return HookOnMessage
 	default:
 		return ""
 	}