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_manifest_validation.py

@@ -9,7 +9,11 @@ from __future__ import annotations
 
 import unittest
 
-from bot_bottle.manifest import ManifestBottle, ManifestIndex
+from unittest.mock import patch
+
+from bot_bottle.env import resolve_env
+from bot_bottle.errors import MissingEnvVarError
+from bot_bottle.manifest import Manifest, ManifestBottle, ManifestIndex
 from bot_bottle.manifest_agent import (
     ManifestAgent,
     ManifestAgentProvider,
@@ -238,5 +242,26 @@ class TestEagerIndexLookups(unittest.TestCase):
         self.assertIsNone(m.git_identity_summary())
 
 
+class TestResolveEnv(unittest.TestCase):
+    """resolve_env raises MissingEnvVarError when an interpolated entry's
+    host var is unset."""
+
+    def _manifest(self, env_dict: dict[str, str]) -> Manifest:
+        idx = ManifestIndex.from_json_obj({
+            "bottles": {"dev": {"env": env_dict}},
+            "agents": {"demo": {"bottle": "dev"}},
+        })
+        return idx.load_for_agent("demo")
+
+    def test_missing_interpolated_host_var_raises(self):
+        manifest = self._manifest({"API_KEY": "${HOST_API_KEY}"})
+        with patch.dict("os.environ", {}, clear=False):
+            import os
+            os.environ.pop("HOST_API_KEY", None)
+            with self.assertRaises(MissingEnvVarError) as cm:
+                resolve_env(manifest)
+        self.assertIn("HOST_API_KEY", str(cm.exception))
+
+
 if __name__ == "__main__":
     unittest.main()