test: add unit tests to pass diff-coverage gate (91%)

Cover the new production lines added on this branch:

- cli/__init__: MissingEnvVarError maps to return 1
- cli/start: no-agents-defined path, prepare_with_preflight calls
  render_preflight, _text_render_preflight backend name output
- env: resolve_env raises MissingEnvVarError for missing interpolated var
- smolmachines/launch: _proxy_host Linux/non-Linux branches,
  _discover_urls git-gate host and supervise URL stamping

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

tests/unit/test_cli_dispatch.py

@@ -64,6 +64,15 @@ class TestMainDispatch(unittest.TestCase):
             main(["x", "a", "b"])
         self.assertEqual([["a", "b"]], seen)
 
+    def test_missing_env_var_error_maps_to_1(self) -> None:
+        from bot_bottle.errors import MissingEnvVarError
+
+        def boom(_rest: list[str]) -> int:
+            raise MissingEnvVarError("MY_VAR", "MY_VAR is not set")
+
+        with patch.dict(climod.COMMANDS, {"x": boom}), patch("sys.stderr", io.StringIO()):
+            self.assertEqual(1, main(["x"]))
+
     def test_manifest_error_maps_to_1(self) -> None:
         def boom(_rest: list[str]) -> int:
             raise ManifestError("bad manifest")