feat(agent): implement the NAT-friendly Classic deployment agent #4

Summary

Implement the Linux host agent and the first home-main deployment adapter.
The agent must work behind home NAT and manage one persistent Classic server on
UDP port 1731 without exposing a remote shell or Docker socket.

Implementation / behavior

• Establish an authenticated outbound WebSocket to the control plane and
  reconnect with bounded exponential backoff.
• Register capabilities and report bounded heartbeat, version, liveness, and
  deployment status.
• Verify target, operation, commit, image repository, and digest against local
  policy before doing work.
• Pull the exact GHCR image, back up server-data, and replace the Compose
  service with graceful shutdown.
• Wait for the container health check and expected server heartbeat.
• Preserve the previous known-good image reference and restore it after a
  failed deployment.
• Keep home-main on UDP 1731 with an independent state directory and QUIC
  identity; never touch the later stable 5.34.x target on UDP 1730.
• Make every operation idempotent and report accepted, started, healthy,
  failed, and rolled-back transitions.

Acceptance criteria

• The agent functions with no inbound TCP port, static IP, or hostname.
• A dropped connection during every lifecycle phase can be recovered without
  running two deployments concurrently.
• Image references are digest-pinned before Compose replacement.
• Backups and data-format-changing rollback behavior are explicit and tested.
• The agent refuses arbitrary commands, paths, environment variables, image
  registries, and unit names from the control plane.
• An installation runbook covers systemd, Docker permissions, GHCR access,
  router UDP forwarding, logs, backups, and recovery.

Dependencies

• Parent:
  feat(control): implement NAT-friendly agent control plane #2
• Control-plane protocol:
  feat(webhook): receive verified GitHub events and persist target state #3

Non-goals

• Do not grant the agent official-server deployment authority.
• Do not execute untrusted PR code on the deployment host.