fix(momus): make keyTrigger specify file-path-only invocation requirement

The previous keyTrigger ('Work plan created → invoke Momus') was too
vague — Sisyphus would fire Momus on inline plans or todo lists,
causing Momus to REJECT because its input_extraction requires exactly
one .sisyphus/plans/*.md file path.

The updated trigger explicitly states:
- Momus should only be invoked when a plan file exists on disk
- The file path must be the sole prompt content
- Inline plans and todo lists should NOT trigger Momus

src/agents/momus.ts

@@ -342,5 +342,6 @@ export const momusPromptMetadata: AgentPromptMetadata = {
     "When user explicitly wants to skip review",
     "For trivial plans that don't need formal review",
   ],
-  keyTrigger: "Work plan created → invoke Momus for review before execution",
+  keyTrigger:
+    "Work plan saved to `.sisyphus/plans/*.md` → invoke Momus with the file path as the sole prompt (e.g. `prompt=\".sisyphus/plans/my-plan.md\"`). Do NOT invoke Momus for inline plans or todo lists.",
 };