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sustained use, exec-channel contention, no SIGWINCH) and is superseded by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

@@ -53,15 +53,16 @@ class TestGitGateGitconfigRender(unittest.TestCase):
         self.assertNotIn("pushInsteadOf", out)
 
     def test_gate_host_can_be_ip_port_form(self):
-        # The smolmachines backend's TSI-allowlisted guest has no
-        # DNS, so it dials git-gate via `<bundle_ip>:<port>`.
+        # A VM backend's guest may have no DNS (e.g. firecracker over
+        # the point-to-point TAP), so it dials git-gate via
+        # `<bundle_ip>:<port>`.
         bottle = fixture_with_git().bottles["dev"]
         out = git_gate_render_gitconfig(bottle.git, "192.168.20.2:9418")
         self.assertIn(
             '[url "git://192.168.20.2:9418/bot-bottle.git"]', out,
         )
 
-    def test_scheme_can_be_http_for_smolmachines(self):
+    def test_scheme_can_be_http_for_vm_backend(self):
         bottle = fixture_with_git().bottles["dev"]
         out = git_gate_render_gitconfig(
             bottle.git, "127.0.0.16:57001", scheme="http",