Arch: stub runtimes in production source tree

[nick-pape]

Category: Code Smell / Architectural Issue

Problem

StubRuntime (583 lines) and StubMcpRuntime live in packages/powerline/src/runtimes/, registered alongside production runtimes in main():

registerRuntime(new StubRuntime());       // Test double
registerRuntime(new StubMcpRuntime());    // Test double
registerRuntime(new GenAIScriptRuntime()); // Real runtime
registerRuntime(new ClaudeCodeRuntime());  // Real runtime

Consequences:

• A client requesting provider: "stub" gets a test harness, not an error
• Stub MCP integration logic is tangled with scenario-execution logic
• 583 lines of test doubles ship in the production bundle
• Test runtimes are indistinguishable from production runtimes to PowerLine's handlers

Proposed Fix

1. Move to packages/powerline/test-fixtures/ or a separate @grackle-ai/runtime-stub package
2. Make registration conditional:

   if (process.env.GRACKLE_ENABLE_TEST_RUNTIMES) {
     registerRuntime(new StubRuntime());
   }

3. Document that stub is for testing only

Impact

Low — works fine today, but test code in prod is a maintenance and security smell.