feat(backend): remove smolmachines; firecracker is the Linux default

Delete the smolmachines backend (the whole bot_bottle/backend/smolmachines
package and its tests). It had fatal Linux issues (TSI networking under
sustained use, exec-channel contention, no SIGWINCH) and is superseded by
the Firecracker backend (issue #342).

Backend selection now:
- default is macos-container on macOS, firecracker on KVM-capable Linux
  hosts, and docker as the last resort (was smolmachines).
- firecracker is selected on a KVM host even when the `firecracker`
  binary isn't installed, so start routes through its preflight and
  prints an install pointer (same UX as require_container), instead of
  silently falling back. Split is_host_capable() (Linux + KVM) out of
  is_available() (adds the binary check) to drive this.

Retarget the cross-backend tests (parity, print-parity, prepare,
workspace, freezer, selection) from smolmachines to firecracker rather
than dropping the coverage. Remove docker.util.image_id/save, which only
smolmachines used. Update README/AGENTS/example bottles and stale
comments; historical docs/prds are left as a point-in-time record.

BREAKING: BOT_BOTTLE_BACKEND=smolmachines now errors as unknown.

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

bot_bottle/backend/docker/cleanup.py

@@ -18,8 +18,7 @@ scan, just as a fallback bucket alongside the project list.
 
 `cleanup` removes everything in the plan.
 
-Active-agent enumeration lives in `backend/docker/enumerate.py`
-(mirror of `backend/smolmachines/enumerate.py`).
+Active-agent enumeration lives in `backend/docker/enumerate.py`.
 """
 
 from __future__ import annotations
@@ -93,8 +92,8 @@ def _list_orphan_state_dirs(
 
     `protected_identities` is the set of slugs that are live in
     ANY backend — used so this docker-side check doesn't reap a
-    running smolmachines bottle's state dir (the layout is shared
-    across both backends)."""
+    running non-docker bottle's state dir (the layout is shared
+    across backends)."""
     state_root = _supervise.bot_bottle_root() / "state"
     if not state_root.is_dir():
         return []
@@ -119,7 +118,7 @@ def prepare_cleanup() -> DockerBottleCleanupPlan:
 
     Pulls the union of live identities across backends via
     `enumerate_active_agents()` so the orphan-state-dir bucket
-    doesn't include slugs whose smolmachines VM is still up."""
+    doesn't include slugs whose non-docker bottle is still up."""
     docker_mod.require_docker()
     projects = list_compose_projects()
     project_set = set(projects)

bot_bottle/backend/docker/enumerate.py

@@ -1,7 +1,6 @@
 """Active-agent enumeration for the docker backend.
 
-Mirrors `backend/smolmachines/enumerate.py`: returns
-`ActiveAgent` records the CLI `list active` command and the
+Returns `ActiveAgent` records the CLI `list active` command and the
 dashboard agents pane consume. Empty when docker isn't reachable
 — gated by `has_backend('docker')` at the cross-backend caller
 so this module trusts that docker is available when called.

bot_bottle/backend/docker/util.py

@@ -167,36 +167,6 @@ def commit_container(container_name: str, image_tag: str) -> None:
     info(f"committed {container_name!r} → {image_tag!r}")
 
 
-def image_id(ref: str) -> str:
-    """Return the content-addressed image ID (e.g.
-    `sha256:abcd...`) for `ref`. The smolmachines backend keys its
-    `.smolmachine` artifact cache on this, so a Dockerfile change
-    that produces a new image automatically invalidates the cache."""
-    r = subprocess.run(
-        ["docker", "image", "inspect", "--format", "{{.Id}}", ref],
-        capture_output=True,
-        text=True,
-        check=False,
-    )
-    if r.returncode != 0:
-        die(
-            f"docker image inspect for {ref!r} failed: "
-            f"{(r.stderr or '').strip() or '<no stderr>'}"
-        )
-    return r.stdout.strip()
-
-
-def save(ref: str, output: str) -> None:
-    """`docker save REF -o OUTPUT`. Writes a tarball of the image
-    layers + manifest to the host path. Used by smolmachines
-    prepare to hand the agent image to a containerized crane that
-    pushes it to the ephemeral registry — bypassing the docker
-    daemon's `docker push` (which on Docker Desktop can't reach a
-    host-loopback registry and refuses plain-HTTP pushes to
-    non-loopback hosts)."""
-    subprocess.run(["docker", "save", ref, "-o", output], check=True)
-
-
 def _silent_run(cmd: Iterable[str]) -> int:
     return subprocess.run(
         list(cmd),