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sustained use, exec-channel contention, no SIGWINCH) and is superseded by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

@@ -99,25 +99,25 @@ class TestOrphanStateDirs(_FakeHomeMixin, unittest.TestCase):
 
     def test_protected_identity_skips_dir(self):
         # `protected_identities` carries slugs that are live in
-        # any backend (smolmachines included). docker's orphan
-        # detection respects them so a running smolmachines
+        # any backend (firecracker included). docker's orphan
+        # detection respects them so a running firecracker
         # bottle's state dir isn't reaped while the VM is up.
-        bottle_state.write_per_bottle_dockerfile("smol-hhh", "FROM x\n")
+        bottle_state.write_per_bottle_dockerfile("fc-hhh", "FROM x\n")
         self.assertEqual(
             [],
-            _list_orphan_state_dirs(set(), {"smol-hhh"}),
+            _list_orphan_state_dirs(set(), {"fc-hhh"}),
         )
 
     def test_protected_overrides_no_live_project(self):
-        # A smolmachines bottle has no docker compose project but
+        # A firecracker bottle has no docker compose project but
         # IS in the protected set; the absence of a project
         # shouldn't cause a reap.
-        bottle_state.write_per_bottle_dockerfile("smol-iii", "FROM x\n")
+        bottle_state.write_per_bottle_dockerfile("fc-iii", "FROM x\n")
         self.assertEqual(
             [],
             _list_orphan_state_dirs(
                 {"bot-bottle-something-else"},  # different project up
-                {"smol-iii"},                       # but smol-iii is live
+                {"fc-iii"},                         # but fc-iii is live
             ),
         )