Probe and report agent feature capabilities (cron, docker, systemd, nginx, etc.) so the panel can gate target pickers without extra round-trips.
Key changes:
- Add agent/internal/capabilities: probes host for features and platform/distro, returns protocol.CapabilitiesMessage cached at agent startup.
- Agent: store probed capabilities, send them on every transport reconnect (sendCapabilities) and include them when using poll transport.
- Protocol: introduce TypeCapabilities and CapabilitiesMessage (map[string]bool + platform/distro fields).
- Poll transport: accept CapabilitiesMessage in Send(), serialize to raw payload and queue for next /poll; support one-shot capabilities in poll body.
- Backend: handle capabilities from socket (on_capabilities) and poll endpoint; persist capabilities in in-memory ConnectedAgent (agent_registry) with helpers get_capabilities/has_capability; include capabilities/platform info in servers API.
- New fleet API (GET /api/v1/fleet/targets): list runnable targets (local + connected agents) and optionally filter agents by a requested feature using the advertised capability map.
Rationale: make agent feature availability explicit and discoverable to the panel so UI pickers can show only applicable agents without asking agents on every UI interaction.
