FEAT-3: policy push from owner to agent

[joy-software]

What

Lets an owner push a new supervision policy to a dial-home agent over the existing order channel, so a policy update reaches an agent that can never be contacted inbound.

Owner (src/spero/owner.py)

• AgentRegistry.queue_policy(agent_id, policy_yaml) appends a {"type": "policy", "policy": ...} order to the agent's queue.
• POST /agents/{agent_id}/policy (async, token-guarded) accepts {"policy": "<yaml>"}, validates it with load_policy_str, returns 422 on invalid YAML or policy, and on success queues the order and returns {"queued": "policy"}.

Agent (src/spero/agent.py)

• latest_policy_order(orders) returns the YAML of the last policy order, so an agent that pulls several at once converges on the newest.
• swap_supervisor(sup, policy_yaml, approver) validates the YAML, stops the old Supervisor, and starts a new one on the same RemoteApprover so approvals keep flowing. An invalid push is logged and skipped, leaving the current supervisor running, so a bad push can never take the agent down.
• The report loop applies approve orders as before, then hot-swaps on a policy order.

Tests

• Owner: policy push queues and delivers the order, then clears it; invalid policy and unparseable YAML both return 422 and queue nothing.
• Agent: latest_policy_order picks the last push; swap_supervisor swaps to the new policy while preserving the approver state; an invalid push leaves the old supervisor running.

Gates

ruff check, ruff format --check, mypy, pytest all green. 234 passed.