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sustained use, exec-channel contention, no SIGWINCH) and is superseded by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

@@ -16,7 +16,7 @@ from bot_bottle import bottle_state
 from bot_bottle import supervise
 from bot_bottle.backend import Bottle, BottleSpec, ExecResult
 from bot_bottle.backend.docker import DockerBottleBackend
-from bot_bottle.backend.smolmachines import SmolmachinesBottleBackend
+from bot_bottle.backend.firecracker import FirecrackerBottleBackend
 from bot_bottle.manifest import ManifestIndex
 
 
@@ -114,13 +114,13 @@ class TestRuntimeWorkspaceProvisioning(_FakeStateMixin, unittest.TestCase):
         bottle.exec.assert_not_called()
         bottle.cp_in.assert_not_called()
 
-    def test_smolmachines_uses_same_running_bottle_method(self) -> None:
-        backend = SmolmachinesBottleBackend()
+    def test_firecracker_uses_same_running_bottle_method(self) -> None:
+        backend = FirecrackerBottleBackend()
         with patch(
-            "bot_bottle.backend.smolmachines.resolve_plan.smolmachines_preflight",
+            "bot_bottle.backend.firecracker.resolve_plan.util.require_firecracker",
         ):
             plan = backend.prepare(
-                _spec(self.tmp, identity="demo-smol-work"),
+                _spec(self.tmp, identity="demo-fc-work"),
                 self.tmp / "stage",
             )
 
@@ -128,10 +128,10 @@ class TestRuntimeWorkspaceProvisioning(_FakeStateMixin, unittest.TestCase):
         backend.provision_workspace(plan, bottle)
 
         bottle.cp_in.assert_called_once_with(str(self.tmp), "/home/node/workspace")
-        metadata = bottle_state.read_metadata("demo-smol-work")
+        metadata = bottle_state.read_metadata("demo-fc-work")
         self.assertIsNotNone(metadata)
         assert metadata is not None
-        self.assertEqual("smolmachines", metadata.backend)
+        self.assertEqual("firecracker", metadata.backend)
 
 
 class TestWorkspaceTrustPath(_FakeStateMixin, unittest.TestCase):