fix: smoke-test agent images after build, add start --no-cache

npm treats optionalDependencies failures as non-fatal, so a transient
network blip fetching claude-code's platform-native binary during
`npm install -g` left a stub CLI in an image that still "built"
successfully — then got baked into the Docker/Container layer cache
until forced to rebuild. Post-build smoke test (provider-declared
argv, run in a throwaway container of the freshly built image) fails
the launch loudly instead of shipping a broken image; --no-cache
gives an escape hatch to force a from-scratch rebuild.

Closes #353.

tests/unit/test_docker_launch_teardown.py

@@ -87,6 +87,7 @@ class TestTeardownWarning(unittest.TestCase):
         buf = io.StringIO()
 
         with mock.patch.object(launch_mod.docker_mod, "build_image"), \
+             mock.patch.object(launch_mod.docker_mod, "verify_agent_image"), \
              mock.patch.object(
                  launch_mod, "egress_tls_init",
                  return_value=(Path("/egress_ca"), Path("/egress_cert")),