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sustained use, exec-channel contention, no SIGWINCH) and is superseded by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

tests/unit/test_docker_util_image.py

@@ -1,4 +1,4 @@
-"""Unit: image_id / tag / push helpers in
+"""Unit: commit_container helper in
 bot_bottle.backend.docker.util (PRD 0023 chunk 4c additions).
 
 Tests mock `subprocess.run` and assert on argv shape + parsing.
@@ -26,47 +26,6 @@ def _fail(stderr: str = "boom") -> subprocess.CompletedProcess:  # type: ignore
     )
 
 
-class TestImageId(unittest.TestCase):
-    def test_strips_trailing_newline(self):
-        # docker image inspect --format ... emits a trailing newline.
-        with patch.object(
-            docker_mod.subprocess, "run",
-            return_value=_ok(stdout="sha256:abcdef\n"),
-        ) as run:
-            self.assertEqual(
-                "sha256:abcdef", docker_mod.image_id("bot-bottle-claude:latest")
-            )
-        argv = run.call_args.args[0]
-        self.assertEqual(
-            ["docker", "image", "inspect", "--format", "{{.Id}}", "bot-bottle-claude:latest"],
-            argv,
-        )
-
-    def test_dies_on_inspect_failure(self):
-        with patch.object(
-            docker_mod.subprocess, "run", return_value=_fail("No such image"),
-        ), patch.object(
-            docker_mod, "die", side_effect=SystemExit("die"),
-        ) as die:
-            with self.assertRaises(SystemExit):
-                docker_mod.image_id("missing:tag")
-            die.assert_called_once()
-            self.assertIn("missing:tag", die.call_args.args[0])
-
-
-class TestSave(unittest.TestCase):
-    def test_save_runs_docker_save(self):
-        with patch.object(
-            docker_mod.subprocess, "run", return_value=_ok(),
-        ) as run:
-            docker_mod.save("bot-bottle-claude:latest", "/tmp/img.tar")
-        argv = run.call_args.args[0]
-        self.assertEqual(
-            ["docker", "save", "bot-bottle-claude:latest", "-o", "/tmp/img.tar"],
-            argv,
-        )
-
-
 class TestCommitContainer(unittest.TestCase):
     def test_runs_docker_commit(self):
         with patch.object(