fix(firecracker): a guest that dies fails its agent-ready waiters at once, with the console tail

[nikhilunni]

Summary

A Firecracker guest that dies before agentd dials ready now fails its waiters immediately, with the console tail in the error, instead of sitting out the 180 s deadline with a generic message.

The problem

Reproduced on a KVM host while debugging the swap snapshot test: the capture guest panicked 3 s after boot, the supervisor pruned the sandbox within 5 s and deleted the jail, but the ready-dial listener task kept the watch sender alive. wait_agent_ready saw neither "ready" nor "closed", waited the full 180 s, and failed with "agentd did not dial ready port". The panic text was only recoverable by polling the jail before the prune.

The change

• The ready-dial listener is owned by the LiveSandbox (AgentReadiness: receiver, task handle, death note).
• Prune and destroy read the last 4 KiB of firecracker.log before removing the jail, store it as the death note, log it at WARN on an unexpected death, and abort the listener so the watch closes.
• A closed watch maps to guest died before agentd dialed ready port with the console tail. The ready fast path and the 180 s backstop for a hung guest are unchanged. start_agent and base capture get the behaviour through their existing calls.
• read_tail is UTF-8 tolerant (a tail that starts mid-character no longer drops the rest).

Tests

• Unit (no KVM): two pending waiters receive the panic marker after the supervisor's real teardown callback; the same with no console file; explicit destroy; mid-character tail.
• Pooled: base capture with a dead guest fails within one second and still destroys the VM.
• KVM (lifecycle.rs, already in the unprivileged FC lane): a fixture whose init exits immediately fails wait_agent_ready within 15 s with "Kernel panic" in the message.

Validation

just check green (2766 tests); Linux cross clippy for firecracker and host-agent clean; Codex adversarial review approved with no findings.

🤖 Generated with Claude Code

https://claude.ai/code/session_01UHxL6gj4o8EvxYpgtwEWaM