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sustained use, exec-channel contention, no SIGWINCH) and is superseded by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

@@ -1,14 +1,14 @@
 """cleanup: stop and remove all orphaned bot-bottle resources.
 
-Walks every registered backend (docker + smolmachines) so a single
-`./cli.py cleanup` reaps both backends' leftovers — orphaned
-smolvm machines won't survive a docker-only cleanup pass (issue
-addressed alongside #77).
+Walks every registered backend (docker, firecracker, macos-container)
+so a single `./cli.py cleanup` reaps every backend's leftovers — a
+firecracker bottle's sidecars won't survive a docker-only cleanup pass
+(issue addressed alongside #77).
 
 Each backend's `prepare_cleanup` enumerates its own resources;
 docker's `_list_orphan_state_dirs` consults
 `enumerate_active_agents()` for the union of live identities so
-state dirs of running smolmachines bottles aren't reaped. State
+state dirs of running non-docker bottles aren't reaped. State
 dirs are shared layout, so docker is the single owner of that
 bucket.
 

bot_bottle/cli/commit.py

@@ -2,10 +2,10 @@
 
 Docker bottles are committed to a local Docker image. Macos-container
 bottles are exported and rebuilt as a local Apple Container image.
-Smolmachines bottles are packed from the running VM into a
-`.smolmachine` artifact. The resulting reference is stored in
-per-bottle state so the next `./cli.py resume <slug>` boots from the
-snapshot instead of rebuilding from the Dockerfile.
+Firecracker bottles stream the guest rootfs out over SSH and rebuild a
+local Docker image. The resulting reference is stored in per-bottle
+state so the next `./cli.py resume <slug>` boots from the snapshot
+instead of rebuilding from the Dockerfile.
 """
 
 from __future__ import annotations

bot_bottle/cli/list.py

@@ -45,8 +45,9 @@ def cmd_list(argv: list[str]) -> int:
             print(name)
         return 0
 
-    # `active` enumerates every backend (docker + smolmachines)
-    # so smolmachines bottles aren't hidden behind the env var.
+    # `active` enumerates every backend (docker, firecracker,
+    # macos-container) so non-docker bottles aren't hidden behind
+    # the env var.
     active = enumerate_active_agents()
     if not active:
         print("no active bot-bottle bottles", file=sys.stderr)

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)