feat(firecracker): add Linux microVM backend to replace smolmachines

Adds a Firecracker-based backend for Linux, providing mature KVM-based
microVM isolation to replace smolmachines/libkrun (issue #342, closes
the dead-end tracked in #332).

Architecture:
- Guest control over SSH (dropbear injected into the rootfs) on a
  point-to-point TAP link. `ssh -t` forwards SIGWINCH natively, so no
  resize bridge is needed.
- Networking: a one-time, root-provisioned pool of user-owned TAP
  devices (no shared bridge → no docker0/virbr0/cni0 collisions) plus a
  dedicated `table inet bot_bottle_fc` nftables table (independent of
  Docker/ufw/firewalld rules). `./cli.py firecracker setup` prints the
  host-appropriate config (NixOS module or sudo script).
- Rootfs: `docker export` → ext4 via `mke2fs -d` (rootless, no mount),
  cached by image digest; per-bottle SSH pubkey + IP passed via the
  kernel cmdline.
- Sidecar: reuses the Docker bundle, published on the slot's host TAP IP.
- Fail-closed isolation: TAP pool verified at preflight; the egress
  boundary is proven empirically post-boot (before the agent runs) by a
  canary probe — the VM must fail to reach the host directly, or launch
  is refused.

Linux hosts with Firecracker + KVM now default to this backend;
macOS stays on macos-container.

Not yet validated end-to-end on live hardware (requires the one-time
network pool). Unit tests + pyright pass.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01G8p32HJgPoS1hLPWubbftM

bot_bottle/backend/firecracker/bottle.py

@@ -0,0 +1,170 @@
+"""Bottle handle for a Firecracker microVM, over SSH.
+
+Every operation routes through SSH to the guest (dropbear, listening on
+the TAP link). `exec` pipes a script over stdin and captures output;
+`cp_in` uses scp; `exec_agent` uses `ssh -t` for an interactive PTY
+session. `ssh -t` forwards the host terminal's SIGWINCH to the remote
+PTY natively, so — unlike the smolmachines backend — no resize bridge
+is needed.
+
+Commands run as the image's `node` user via `runuser`, with HOME/USER/
+PATH and the bottle env (HTTPS_PROXY at the sidecar, CA paths, …) set
+per-invocation through `env` (the VM itself just runs the init; it has
+no baked-in process env like a `docker run` container would).
+"""
+
+from __future__ import annotations
+
+import os
+import shlex
+import subprocess
+import sys
+from pathlib import Path
+from typing import Mapping, cast
+
+from ...agent_provider import PromptMode, prompt_args
+from .. import Bottle, ExecResult
+from ..terminal import exec_shell_script
+from . import util
+
+
+_HOME_FOR = {"node": "/home/node", "root": "/root"}
+_DEFAULT_PATH_FOR = {
+    "node": (
+        "/home/node/.local/bin:"
+        "/home/node/.codex/packages/standalone/current/bin:"
+        "/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin"
+    ),
+    "root": "/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin",
+}
+
+
+def _env_assignments_for(user: str, env: Mapping[str, str]) -> list[str]:
+    home = _HOME_FOR.get(user, f"/home/{user}")
+    out = [f"HOME={home}", f"USER={user}"]
+    if "PATH" not in env:
+        out.append(f"PATH={_DEFAULT_PATH_FOR.get(user, _DEFAULT_PATH_FOR['root'])}")
+    for k, v in env.items():
+        out.append(f"{k}={v}")
+    return out
+
+
+class FirecrackerBottle(Bottle):
+    def __init__(
+        self,
+        name: str,
+        *,
+        private_key: Path,
+        guest_ip: str,
+        guest_env: Mapping[str, str] | None = None,
+        prompt_path_in_guest: str | None = None,
+        agent_command: str = "claude",
+        agent_prompt_mode: PromptMode = "append_file",
+        agent_provider_template: str = "claude",
+        terminal_title: str = "",
+        terminal_color: str = "",
+        agent_workdir: str = "/home/node",
+    ) -> None:
+        self.name = name
+        self._private_key = private_key
+        self._guest_ip = guest_ip
+        self._guest_env = dict(guest_env or {})
+        self.prompt_path = prompt_path_in_guest
+        self._agent_prompt_mode = agent_prompt_mode
+        self.agent_command = agent_command
+        self.terminal_title = terminal_title
+        self.terminal_color = terminal_color
+        self.agent_provider_template = agent_provider_template
+        self.agent_workdir = agent_workdir
+
+    def _ssh(self, *, tty: bool) -> list[str]:
+        argv = util.ssh_base_argv(self._private_key, self._guest_ip)
+        if tty:
+            # -t: allocate a remote PTY and forward window-size changes.
+            argv.insert(1, "-t")
+        return argv
+
+    def _agent_remote_argv(self, argv: list[str]) -> list[str]:
+        """The `runuser … <agent> …` command run inside the guest."""
+        full_argv = list(argv)
+        full_argv.extend(
+            prompt_args(
+                cast(PromptMode, self._agent_prompt_mode),
+                self.prompt_path,
+                argv=full_argv,
+            )
+        )
+        remote = ["runuser", "-u", "node", "--",
+                  "env", *_env_assignments_for("node", self._guest_env)]
+        if self.agent_workdir and self.agent_workdir != _HOME_FOR["node"]:
+            remote += ["sh", "-lc", f"cd {self.agent_workdir} && exec \"$@\"",
+                       "bot-bottle-agent"]
+        remote += [self.agent_command, *full_argv]
+        return remote
+
+    def agent_argv(self, argv: list[str], *, tty: bool = True) -> list[str]:
+        return [*self._ssh(tty=tty), "--", *self._agent_remote_argv(argv)]
+
+    def exec_agent(self, argv: list[str], *, tty: bool = True) -> int:
+        agent_argv = self.agent_argv(argv, tty=tty)
+        script = (
+            exec_shell_script(agent_argv, self.terminal_title, self.terminal_color)
+            if tty else None
+        )
+        if script is None:
+            return subprocess.run(agent_argv, check=False).returncode
+        return subprocess.run(["sh", "-lc", script], check=False).returncode
+
+    def exec(self, script: str, *, user: str = "node") -> ExecResult:
+        # Pipe the script over stdin (`sh -s`) so nothing needs shell
+        # quoting through the SSH command line.
+        remote = ["runuser", "-u", user, "--",
+                  "env", *_env_assignments_for(user, self._guest_env),
+                  "/bin/sh", "-s"]
+        result = subprocess.run(
+            [*self._ssh(tty=False), "--", *remote],
+            input=script, capture_output=True, text=True, check=False,
+        )
+        return ExecResult(
+            returncode=result.returncode,
+            stdout=result.stdout,
+            stderr=result.stderr,
+        )
+
+    def cp_in(self, host_path: str, container_path: str) -> None:
+        # Stream a tar over the SSH exec channel rather than scp: the
+        # guest runs dropbear, which ships no sftp-server/scp, but every
+        # agent image has `tar`. Copy so `container_path` becomes a copy
+        # of `host_path` (docker cp semantics — callers rm -rf the
+        # destination first).
+        host_parent = os.path.dirname(host_path.rstrip("/")) or "/"
+        host_base = os.path.basename(host_path.rstrip("/"))
+        guest_parent = os.path.dirname(container_path.rstrip("/")) or "/"
+        guest_base = os.path.basename(container_path.rstrip("/"))
+        remote = (
+            f"mkdir -p {shlex.quote(guest_parent)} && "
+            f"cd {shlex.quote(guest_parent)} && "
+            f"rm -rf {shlex.quote(guest_base)} && tar -xf - && "
+            f"{{ [ {shlex.quote(host_base)} = {shlex.quote(guest_base)} ] || "
+            f"mv {shlex.quote(host_base)} {shlex.quote(guest_base)}; }}"
+        )
+        tar = subprocess.Popen(
+            ["tar", "-C", host_parent, "-cf", "-", host_base],
+            stdout=subprocess.PIPE,
+        )
+        ssh = subprocess.run(
+            [*self._ssh(tty=False), "--", "sh", "-c", remote],
+            stdin=tar.stdout, capture_output=True, text=True, check=False,
+        )
+        tar.wait()
+        if tar.returncode != 0 or ssh.returncode != 0:
+            sys.stderr.write(ssh.stderr)
+            raise subprocess.CalledProcessError(
+                ssh.returncode or tar.returncode,
+                ["cp_in", host_path, container_path],
+            )
+
+    def close(self) -> None:
+        # Real teardown (VM terminate, TAP release) lives on the launch
+        # ExitStack; this is the idempotent alias the ABC expects.
+        pass