fix: remove CLI backend assumptions and add docker fallback prompt

- Remove hardcoded --backend=macos-container flag reference in
  firecracker/util.py require_firecracker() error message
- Remove --backend flag from cli.py start; backend selection now
  driven exclusively by BOT_BOTTLE_BACKEND env var or auto-selection
- Skip unavailable backends in cli.py cleanup (fixes crash on Linux
  when macos-container.prepare_cleanup calls require_container())
- Add two-tier auto-selection: VM backend first (macos-container on
  macOS, firecracker on Linux+KVM); fall back to docker with a
  security warning and interactive i/d/q prompt; exit if docker
  also unavailable and print VM install instructions

Closes #344

tests/unit/test_cli_start_backend_flag.py

@@ -1,61 +1,29 @@
-"""Unit: `cli.py start --backend=<name>` flag (issue #77).
+"""Unit: backend resolution priority — explicit name > env var.
 
-Asserts that the flag wins over the env var, that the env var is
-the fallback, and that the choices are pulled from the backend
-registry (so adding a backend lights up in argparse without code
-edits)."""
+The `--backend` flag has been removed from `cli.py start`; backend
+selection is driven by BOT_BOTTLE_BACKEND or auto-selection only.
+`get_bottle_backend` still accepts an explicit name so that `resume`
+(which records the backend from a prior session) and the `backend`
+sub-command can pass one directly."""
 
 from __future__ import annotations
 
-import argparse
 import os
 import unittest
 from unittest.mock import patch
 
 
-from bot_bottle.backend import known_backend_names
-
-
-class TestStartBackendFlag(unittest.TestCase):
-    """The flag is wired by `cmd_start`'s argparse and threaded
-    through `prepare_with_preflight(backend_name=...)`. Rather than
-    drive the whole start flow (which builds containers), we test
-    the argparse shape and the resolution function separately."""
-
-    def _build_parser(self):
-        # Mirror the parser definition from `cmd_start` so this
-        # test doesn't have to invoke the full command.
-        parser = argparse.ArgumentParser(prog="cb start")
-        parser.add_argument(
-            "--backend",
-            choices=known_backend_names(),
-            default=None,
-        )
-        parser.add_argument("name")
-        return parser
-
-    def test_flag_recognized(self):
-        args = self._build_parser().parse_args(["--backend=firecracker", "researcher"])
-        self.assertEqual("firecracker", args.backend)
-        self.assertEqual("researcher", args.name)
-
-    def test_flag_default_none_means_env_or_default_backend(self):
-        args = self._build_parser().parse_args(["researcher"])
-        self.assertIsNone(args.backend)
-
-    def test_invalid_backend_rejected_by_argparse(self):
-        parser = self._build_parser()
-        with self.assertRaises(SystemExit):
-            parser.parse_args(["--backend=garbage", "researcher"])
-
-    def test_resolution_priority_explicit_over_env(self):
-        # Independent assertion that get_bottle_backend (where
-        # `--backend` ultimately threads to) prefers the explicit
-        # name over BOT_BOTTLE_BACKEND.
+class TestBackendResolutionPriority(unittest.TestCase):
+    def test_explicit_name_overrides_env_var(self):
         from bot_bottle.backend import get_bottle_backend
         with patch.dict(os.environ, {"BOT_BOTTLE_BACKEND": "firecracker"}):
             self.assertEqual("docker", get_bottle_backend("docker").name)
 
+    def test_env_var_used_when_no_explicit_name(self):
+        from bot_bottle.backend import get_bottle_backend
+        with patch.dict(os.environ, {"BOT_BOTTLE_BACKEND": "firecracker"}):
+            self.assertEqual("firecracker", get_bottle_backend().name)
+
 
 if __name__ == "__main__":
     unittest.main()