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sustained use, exec-channel contention, no SIGWINCH) and is superseded by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

@@ -34,7 +34,7 @@ class TestPalettePrintf(unittest.TestCase):
 
 
 class TestExecShellScript(unittest.TestCase):
-    _ARGV = ["smolvm", "machine", "exec", "--name", "x", "--", "claude"]
+    _ARGV = ["ssh", "-t", "100.64.0.1", "--", "claude"]
 
     def test_no_decoration_returns_none(self):
         self.assertIsNone(exec_shell_script(self._ARGV))
@@ -58,7 +58,7 @@ class TestExecShellScript(unittest.TestCase):
         self.assertIn("\\033]111", script)  # background reset
         # No exec-replace when palette is active (shell must survive for reset)
         parts = script.split("; ")
-        agent_part = next(p for p in parts if "smolvm" in p)
+        agent_part = next(p for p in parts if "ssh" in p)
         self.assertFalse(agent_part.startswith("exec "))
 
     def test_title_and_color_both_appear(self):