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sustained use, exec-channel contention, no SIGWINCH) and is superseded by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/cli/supervise.py

@@ -28,9 +28,6 @@ from ..backend.docker.egress_apply import (
 from ..backend.macos_container.egress_apply import (
     applicator as _macos_applicator,
 )
-from ..backend.smolmachines.egress_apply import (
-    applicator as _smolmachines_applicator,
-)
 from ..log import Die, error, info
 
 from ..supervise import (
@@ -79,8 +76,6 @@ def apply_routes_change(slug: str, content: str) -> tuple[str, str]:
     backend = meta.backend if meta is not None else ""
     if backend == "macos-container":
         return _macos_applicator.apply_routes_change(slug, content)
-    if backend == "smolmachines":
-        return _smolmachines_applicator.apply_routes_change(slug, content)
     return _docker_applicator.apply_routes_change(slug, content)