Add executor tool loop (A12/A15) — step 5b

- tools.py: four flat tools (read_file, write_file, list_dir,
  run_command) per A15's reduced manifest. Deliberately unsandboxed:
  the VM is the trust boundary (B3) and tool use is auto-approved
  inside an approved unit of work (A4), so path confinement would add
  failure modes without adding safety. What is enforced is
  operational — timeout with process-group kill so a backgrounded
  server cannot outlive its command, output truncation so a build log
  cannot eat the context window, closed stdin so prompts fail fast,
  and a scrubbed environment so project code never sees the agent's
  credentials
- executor.py: renders the log, calls the executor role with the
  reduced manifest, dispatches tool calls, and checkpoints every
  result (A13). Escalation is observable, never self-assessed (A12):
  consecutive command failures and a hard turn cap. Failed reads do
  not count — looking around is not failing
- Until delegation exists (step 7), escalation gates to the human
  with the failures attached
- control.py: start milestone now runs the executor; a stopped
  milestone gates to AWAIT_INPUT

Two bugs found by running it live against real models:

- A turn-ending gate tool (submit_plan) left a tool_call with no
  tool_result, which providers reject outright, so every planning
  turn after the first failed with a 400. The planner now records the
  gate as the call's outcome, which is also the truthful transcript
- The renderer now emits a placeholder for any tool_call lacking a
  result, so a crash between call and result (A3) cannot poison the
  transcript permanently

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

nisse-dev/tests/test_render.py

@@ -71,6 +71,33 @@ def test_compaction_replaces_its_range_only() -> None:
     assert contents == ["kept", "[summary of earlier conversation]\nthe old stuff"]
 
 
+def test_tool_call_without_a_result_gets_a_placeholder() -> None:
+    # Providers reject a dangling tool_use; this happens after a crash (A3)
+    # or with turn-ending gate tools like submit_plan.
+    events = stored(
+        [
+            UserMessage(text="go", source="m:1"),
+            ToolCall(call_id="call-x", tool="submit_plan", arguments={}),
+        ]
+    )
+    body = OpenAIChatRenderer().render(events, tools=None, model="q")
+    assert [m["role"] for m in body["messages"]] == ["user", "assistant", "tool"]
+    assert body["messages"][2]["tool_call_id"] == "call-x"
+    assert body["messages"][2]["content"] == "[no result recorded]"
+
+
+def test_answered_tool_call_gets_no_placeholder() -> None:
+    events = stored(
+        [
+            ToolCall(call_id="call-x", tool="run_command", arguments={}),
+            ToolResult(call_id="call-x", result="ok", success=True),
+        ]
+    )
+    body = OpenAIChatRenderer().render(events, tools=None, model="q")
+    assert [m["role"] for m in body["messages"]] == ["assistant", "tool"]
+    assert body["messages"][1]["content"] == "ok"
+
+
 def test_gate_event_renders_as_context() -> None:
     events = stored([GateOpened(gate="arch", reason="plan submitted (rev-1)")])
     body = OpenAIChatRenderer().render(events, tools=None, model="q")