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sustained use, exec-channel contention, no SIGWINCH) and is superseded by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

@@ -83,9 +83,9 @@ class TestCmdCommitSlugArg(_FakeHomeMixin, unittest.TestCase):
         mock_gf.assert_called_once_with("macos-container")
         mock_freezer.commit_slug.assert_called_once_with(slug)
 
-    def test_commits_smolmachines_bottle(self):
+    def test_commits_firecracker_bottle(self):
         slug = "dev-abc12"
-        self._write_meta(slug, "smolmachines")
+        self._write_meta(slug, "firecracker")
 
         with patch("bot_bottle.cli.commit.get_freezer") as mock_gf:
             mock_freezer = MagicMock()
@@ -93,7 +93,7 @@ class TestCmdCommitSlugArg(_FakeHomeMixin, unittest.TestCase):
             rc = cmd_commit([slug])
 
         self.assertEqual(0, rc)
-        mock_gf.assert_called_once_with("smolmachines")
+        mock_gf.assert_called_once_with("firecracker")
 
     def test_returns_zero_on_commit_cancelled(self):
         slug = "dev-abc12"