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sustained use, exec-channel contention, no SIGWINCH) and is superseded by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

@@ -241,7 +241,7 @@ class TestBottleMetadataBackend(_FakeHomeMixin, unittest.TestCase):
         assert loaded is not None
         self.assertEqual("docker", loaded.backend)
 
-    def test_backend_field_roundtrips_smolmachines(self):
+    def test_backend_field_roundtrips_firecracker(self):
         meta = BottleMetadata(
             identity="dev-b2",
             agent_name="dev",
@@ -249,13 +249,13 @@ class TestBottleMetadataBackend(_FakeHomeMixin, unittest.TestCase):
             copy_cwd=False,
             started_at="2026-06-02T00:00:00+00:00",
             compose_project="",
-            backend="smolmachines",
+            backend="firecracker",
         )
         write_metadata(meta)
         loaded = read_metadata("dev-b2")
         self.assertIsNotNone(loaded)
         assert loaded is not None
-        self.assertEqual("smolmachines", loaded.backend)
+        self.assertEqual("firecracker", loaded.backend)
 
     def test_missing_backend_field_defaults_to_empty(self):
         # Old state dirs written before PRD 0040 have no backend key.