fix(dog): set BD_ACTOR environment variable for dog sessions

Dogs spawned by gt dog dispatch couldn't find their mail because BD_ACTOR
wasn't set in their tmux environment. The AgentEnv function had no case for
the "dog" role.

Add "dog" case to AgentEnv that sets:
- BD_ACTOR=deacon/dogs/<name>
- GIT_AUTHOR_NAME=dog-<name>
- GIT_AUTHOR_EMAIL=dog-<name>@gastown.local

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

internal/config/env.go

@@ -61,6 +61,11 @@ func AgentEnv(cfg AgentEnvConfig) map[string]string {
 		env["GIT_AUTHOR_NAME"] = "boot"
 		env["GIT_AUTHOR_EMAIL"] = "boot@gastown.local"
 
+	case "dog":
+		env["BD_ACTOR"] = fmt.Sprintf("deacon/dogs/%s", cfg.AgentName)
+		env["GIT_AUTHOR_NAME"] = fmt.Sprintf("dog-%s", cfg.AgentName)
+		env["GIT_AUTHOR_EMAIL"] = fmt.Sprintf("dog-%s@gastown.local", cfg.AgentName)
+
 	case "witness":
 		env["GT_RIG"] = cfg.Rig
 		env["BD_ACTOR"] = fmt.Sprintf("%s/witness", cfg.Rig)