[agent] Add blue/green host daemon binary rollout with systemd recovery fallback

[Copilot]

This change adds a resilient host-agent upgrade path: keep a last-known-good binary on-host and recover automatically if the daemon fails after an update. The daemon now runs through stable symlinks so rollout and rollback are atomic.

• Blue/green host binary slots + stable pointers

  • unbounded-agent-install.sh now alternates installs between:
    • /usr/local/bin/unbounded-agent-blue
    • /usr/local/bin/unbounded-agent-green
  • Maintains:
    • /usr/local/bin/unbounded-agent-current (active target)
    • /usr/local/bin/unbounded-agent-last-good (rollback target)
  • Improved archive download/extract error handling for upgrade failures.

• Daemon unit switched to indirection + failure hook

  • unbounded-agent-daemon.service now executes:
    • /usr/local/bin/unbounded-agent-current daemon
  • Adds:
    • OnFailure=unbounded-agent-daemon-recovery.service

• Systemd recovery path for self-healing rollback

  • New recovery unit + script:
    • unbounded-agent-daemon-recovery.service
    • unbounded-agent-daemon-recovery.sh
  • Recovery behavior:
    • repoint unbounded-agent-current to unbounded-agent-last-good
    • systemctl reset-failed unbounded-agent-daemon.service
    • restart daemon

• Lifecycle cleanup alignment

  • Reset/uninstall now removes/disables:
    • recovery unit
    • recovery script
    • blue/green/current/last-good binary artifacts
  • Added constants for recovery unit/script paths and wired host phase to install all daemon recovery assets.

• Script contract updates

  • Provision script tests updated to assert blue/green/current/last-good and recovery artifact expectations.

# unbounded-agent-daemon.service
[Service]
ExecStart=/usr/local/bin/unbounded-agent-current daemon
OnFailure=unbounded-agent-daemon-recovery.service