feat(config): expand role_registry + fix T4 default runtime

Role registry additions:
- T2: add ai, security, mobile domains
- T3: add data, ai, security, mobile, database, devops, docs domains
- T4: add data (data-engineer), embedded (firmware-engineer)
- T5: add accessibility, e2e, frontend, data verifier roles

TeamRunner consistency fix:
- T4 briefs now default to 'coding_agent' runtime (Claude Code)
  per build spec: 'Claude Code as default T4 runtime'
  T3 can still override preferred_runtime per task

core/team_runner.py

@@ -554,7 +554,9 @@ class TeamRunner:
         briefs: list[TaskBrief] = []
         for task in tasks:
             role = task.get("role", "default")
-            pref_runtime = task.get("preferred_runtime", "standard")
+            # T4 is the coding/implementation tier; default to coding_agent
+            # so implementers use Claude Code unless T3 explicitly overrides.
+            pref_runtime = task.get("preferred_runtime", "coding_agent")
             brief = parent.make_child_brief(
                 tier=4,
                 role=role,