fix(dog): properly set identity for dog sessions

Three fixes to make dog dispatch work end-to-end:

1. Add BuildDogStartupCommand in loader.go
   - Similar to BuildPolecatStartupCommand/BuildCrewStartupCommand
   - Passes AgentName to AgentEnv so BD_ACTOR is exported in startup command

2. Use BuildDogStartupCommand in dog.go
   - Removes ineffective SetEnvironment calls (env vars set after shell starts
     don't propagate to already-running processes)

3. Add "dog" case in mail_identity.go detectSenderFromRole
   - Dogs now use BD_ACTOR for mail identity
   - Without this, dogs fell through to "overseer" and couldn't find their mail

Tested: dog alpha now correctly sees inbox as deacon/dogs/alpha

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

internal/cmd/mail_identity.go

@@ -129,6 +129,13 @@ func detectSenderFromRole(role string) string {
 			return fmt.Sprintf("%s/refinery", rig)
 		}
 		return detectSenderFromCwd()
+	case "dog":
+		// Dogs use BD_ACTOR directly (set by BuildDogStartupCommand)
+		actor := os.Getenv("BD_ACTOR")
+		if actor != "" {
+			return actor
+		}
+		return detectSenderFromCwd()
 	default:
 		// Unknown role, try cwd detection
 		return detectSenderFromCwd()