feat: agent definition v2 (registry, persistence, developerInstructions channel)

Registry: POST /tasks accepts agent_name, loading <name>.json
from repo-root agents/ (traversal-guarded, skill-pack re-validated);
agent and agent_name together is a 400. Persistence: tasks gains
agent_name/agent_version/agent_instructions (migrate + PG schema),
returned by GET /tasks; stored prompt stays clean. Channel: local
and docker dispatch pass --developer-instructions to host.py
dispatch -> create_thread(developerInstructions=...); run/fork do
not re-send. Cloud executor has no such channel, so the worker
falls back to the v1 marked header in the in-memory dispatch
prompt only (DB prompt stays clean). Goal/coordinator tasks
still reject instructions.

Tests: test_agent_def_v2.py (7 tests) + updated v1 suite.

cloud/db.py

@@ -880,6 +880,9 @@ POSTGRES_SCHEMA = (
     "parent_task_id TEXT, "
     "coordinator_part TEXT, "
     "goal_resume_pending INTEGER DEFAULT 0, "
+    "agent_name TEXT, "
+    "agent_version TEXT, "
+    "agent_instructions TEXT, "
     "UNIQUE(project_id, idempotency_key))",
     "CREATE INDEX IF NOT EXISTS idx_tasks_status ON tasks(status)",
     # 附件包与任务附件：与 server._ensure_uploads_tables 同款，