docs: add ADR-0021 remote operations API

coding-agent/data/eval/golden/parent-child-subagent-001.jsonl

@@ -0,0 +1,7 @@
+{"id":"parent-user","kind":"message","payload":{"role":"user","content":"parent task"},"timestamp":1700000101.0}
+{"id":"parent-tool-call","kind":"tool_call","payload":{"id":"tc-subagent","name":"subagent","arguments":{"goal":"child task"}},"timestamp":1700000102.0}
+{"id":"child-user","kind":"message","payload":{"role":"user","content":"child task"},"timestamp":1700000103.0,"meta":{"skip_context":true,"subagent_child":true}}
+{"id":"child-assistant","kind":"message","payload":{"role":"assistant","content":"child done"},"timestamp":1700000104.0,"meta":{"skip_context":true,"subagent_child":true}}
+{"id":"child-note","kind":"message","payload":{"role":"assistant","content":"child hidden note"},"timestamp":1700000105.0,"meta":{"skip_context":true,"subagent_child":true}}
+{"id":"parent-tool-result","kind":"tool_result","payload":{"tool_call_id":"tc-subagent","content":"Subagent completed: child done"},"timestamp":1700000106.0}
+{"id":"parent-assistant","kind":"message","payload":{"role":"assistant","content":"parent done"},"timestamp":1700000107.0}

coding-agent/data/eval/golden/parent-child-subagent-001.yaml

@@ -0,0 +1,11 @@
+task: "Run a child task and report back to the parent"
+
+expected_tools:
+  - name: "subagent"
+    input_parameters:
+      goal: "child task"
+
+forbidden_tools:
+  - "bash_run"
+
+threshold: 1.0