feat(invariants): implement package runtime checks

packages/subagent/subagent-inprocess/src/structured-protocol.ts

@@ -0,0 +1,10 @@
+/** Model-facing constants shared by structured child execution and its invariant. */
+
+/** The model-facing tool name a structured child must call to finish. */
+export const STRUCTURED_OUTPUT_TOOL = 'structured_output'
+
+/** The terminal structured-result instruction appended to a child request. */
+export const STRUCTURED_OUTPUT_INSTRUCTION
+  = 'When you have your final answer, you MUST report it by calling the '
+    + `\`${STRUCTURED_OUTPUT_TOOL}\` tool with arguments matching its parameter schema exactly. `
+    + 'Do not finish with a plain text answer: only the tool call counts as your result.'