RFC 0001-P3: Multi-agent plumbing + Agents UI

Phase 3: real multi-agent support on the server side, plus the UI surface for managing agents.

## Acceptance criteria

- Real `agentregistry` service on the server: per-agent control-channel registry with sticky routing; rebalancing on rediscovery.
- `queue_message.agent_id` populated at enqueue time from `device.agent_id`.
- New UI page at `client/src/protoFleet/views/Agents/`, modeled on the Pools view: list connected agents, last-seen, version, owned device counts, basic health.
- New `AgentManagementService` proto + handler: `ListAgents`, `GetAgentHealth`, `RotateAgentKey`, `RotateAgentKeypair`.
- `RotateAgentKey`: revokes the existing api_key and issues a new one over an authenticated `ControlStream` (bearer-token rotation only).
- `RotateAgentKeypair`: agent generates a new keypair locally; existing keys re-wrapped to new pubkey via the existing crypto delivery path.

## Reference

RFC 0001 Phase 3: https://github.com/block/proto-fleet/pull/145