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sustained use, exec-channel contention, no SIGWINCH) and is superseded by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)