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sustained use, exec-channel contention, no SIGWINCH) and is superseded by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

@@ -35,8 +35,8 @@ class TestStartBackendFlag(unittest.TestCase):
         return parser
 
     def test_flag_recognized(self):
-        args = self._build_parser().parse_args(["--backend=smolmachines", "researcher"])
-        self.assertEqual("smolmachines", args.backend)
+        args = self._build_parser().parse_args(["--backend=firecracker", "researcher"])
+        self.assertEqual("firecracker", args.backend)
         self.assertEqual("researcher", args.name)
 
     def test_flag_default_none_means_env_or_default_backend(self):
@@ -53,7 +53,7 @@ class TestStartBackendFlag(unittest.TestCase):
         # `--backend` ultimately threads to) prefers the explicit
         # name over BOT_BOTTLE_BACKEND.
         from bot_bottle.backend import get_bottle_backend
-        with patch.dict(os.environ, {"BOT_BOTTLE_BACKEND": "smolmachines"}):
+        with patch.dict(os.environ, {"BOT_BOTTLE_BACKEND": "firecracker"}):
             self.assertEqual("docker", get_bottle_backend("docker").name)