feat(firecracker): persistent infra-VM singleton lifecycle (Stage B, 4/n)

The infra VM must outlive the short-lived `start` launcher and be reused
across launches. Add an idempotent singleton:

- `ensure_running()` adopts the infra VM when its control plane is already
  healthy (a prior launcher booted it), else clears any stale VM and boots
  a fresh one. Returns a handle usable for CA fetch / git-gate provisioning
  whether we booted it or adopted it.
- boot is `detached` (firecracker in its own session via start_new_session)
  so it survives the launcher exiting; its PID is recorded so a later
  process can `stop()` it. `_kill_pidfile` SIGTERM/SIGKILLs but only if the
  PID is still a firecracker process (guards a recycled PID).
- a STABLE SSH key (generated once under the infra cache dir, re-injected
  each boot via the cmdline) so any launcher can SSH in to fetch the CA /
  provision, not just the one that booted the VM. `InfraVm.vm` is None in
  the adopted case; teardown then goes through the PID file.

Verified on a KVM host: first ensure_running boots; a second adopts it
(same PID, no reboot) and can still reach /health and fetch the gateway CA
over SSH; stop() tears it down (control plane then unreachable).

Next: git-gate provisioning into the VM over SSH (today docker exec/cp),
then swap consolidated_launch.py onto the infra VM.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01WBMWTEtQdJ4W5UrWuLHCck

bot_bottle/backend/firecracker/firecracker_vm.py

@@ -118,9 +118,15 @@ def boot(
     vcpus: int = 2,
     mem_mib: int = 2048,
     guest_mac: str = "06:00:AC:10:00:02",
+    detached: bool = False,
 ) -> VmHandle:
     """Write the config and launch the VMM. Returns once the process is
-    spawned; callers wait for SSH readiness separately."""
+    spawned; callers wait for SSH readiness separately.
+
+    `detached` starts the VMM in its own session (`start_new_session`) so it
+    survives the launcher exiting — used for the persistent per-host infra
+    VM, which must outlive the short-lived `start` process (agent VMs stay
+    attached and are torn down with the launcher)."""
     run_dir.mkdir(parents=True, exist_ok=True)
     config_path = run_dir / "config.json"
     console_log = run_dir / "console.log"
@@ -137,6 +143,7 @@ def boot(
     process = subprocess.Popen(
         ["firecracker", "--no-api", "--config-file", str(config_path)],
         stdout=log_fh, stderr=subprocess.STDOUT, stdin=subprocess.DEVNULL,
+        start_new_session=detached,
     )
     return VmHandle(process=process, guest_ip=guest_ip, console_log=console_log)