fix: #163 — remove [turn N] suffix pollution from run_turn_loop; file #164 timeout-cancellation followup

#163: run_turn_loop no longer injects f'{prompt} [turn N]' into follow-up
prompts. The suffix was never defined or interpreted anywhere — not by the
engine, not by the system prompt, not by any LLM. It looked like a real
user-typed annotation in the transcript and made replay/analysis fragile.

New behaviour:
- turn 0 submits the original prompt (unchanged)
- turn > 0 submits caller-supplied continuation_prompt if provided, else
  the loop stops cleanly — no fabricated user turn
- added continuation_prompt: str | None = None parameter to run_turn_loop
- added --continuation-prompt CLI flag for claws scripting multi-turn loops
- zero '[turn' strings ever appear in mutable_messages or stdout now

Behaviour change for existing callers:
- Before: run_turn_loop(prompt, max_turns=3) submitted 3 turns
  ('prompt', 'prompt [turn 2]', 'prompt [turn 3]')
- After:  run_turn_loop(prompt, max_turns=3) submits 1 turn ('prompt')
- To preserve old multi-turn behaviour, pass continuation_prompt='Continue.'
  or any structured follow-up text

One existing timeout test (test_budget_is_cumulative_across_turns) updated
to pass continuation_prompt so the cumulative-budget contract is actually
exercised across turns instead of trivially satisfied by a one-turn loop.

#164 filed: addresses reviewer feedback on #161. The wall-clock timeout
bounds the caller-facing wait, but the underlying submit_message worker
thread keeps running and can mutate engine state after the timeout
TurnResult is returned. A cooperative cancel_event pattern is sketched in
the pinpoint; real asyncio.Task.cancel() support will come once provider
IO is async-native (larger refactor).

Tests (tests/test_run_turn_loop_continuation.py, 8 tests):
- TestNoTurnSuffixInjection (2): zero '[turn' strings in any submitted
  prompt, both default and explicit-continuation paths
- TestContinuationDefaultStopsAfterTurnZero (2): default loops run exactly
  one turn; engine.submit_message called exactly once despite max_turns=10
- TestExplicitContinuationBehaviour (2): turn 0 = original, turn N = continuation
  verbatim; max_turns still respected
- TestCLIContinuationFlag (2): CLI default emits only '## Turn 1';
  --continuation-prompt wires through to multi-turn behaviour

Full suite: 67/67 passing.

Closes ROADMAP #163. Files #164.

tests/test_run_turn_loop_continuation.py

@@ -0,0 +1,161 @@
+"""Tests for run_turn_loop continuation contract (ROADMAP #163).
+
+The deprecated ``f'{prompt} [turn N]'`` suffix injection is gone. Verifies:
+- No ``[turn N]`` string ever lands in a submitted prompt
+- Default (``continuation_prompt=None``) stops the loop after turn 0
+- Explicit ``continuation_prompt`` is submitted verbatim on subsequent turns
+- The first turn always gets the original prompt, not the continuation
+"""
+
+from __future__ import annotations
+
+import subprocess
+import sys
+from pathlib import Path
+from unittest.mock import patch
+
+sys.path.insert(0, str(Path(__file__).resolve().parent.parent))
+
+from src.models import UsageSummary  # noqa: E402
+from src.query_engine import TurnResult  # noqa: E402
+from src.runtime import PortRuntime  # noqa: E402
+
+
+def _completed_result(prompt: str) -> TurnResult:
+    return TurnResult(
+        prompt=prompt,
+        output='ok',
+        matched_commands=(),
+        matched_tools=(),
+        permission_denials=(),
+        usage=UsageSummary(),
+        stop_reason='completed',
+    )
+
+
+class TestNoTurnSuffixInjection:
+    """Core acceptance: no prompt submitted to the engine ever contains '[turn N]'."""
+
+    def test_default_path_submits_original_prompt_only(self) -> None:
+        runtime = PortRuntime()
+        submitted: list[str] = []
+
+        def _capture(prompt, commands, tools, denials):
+            submitted.append(prompt)
+            return _completed_result(prompt)
+
+        with patch('src.runtime.QueryEnginePort.from_workspace') as mock_factory:
+            engine = mock_factory.return_value
+            engine.submit_message.side_effect = _capture
+
+            runtime.run_turn_loop('investigate this bug', max_turns=3)
+
+            # Without continuation_prompt, only turn 0 should run
+            assert submitted == ['investigate this bug']
+            # And no '[turn N]' suffix anywhere
+            for p in submitted:
+                assert '[turn' not in p, f'found [turn suffix in submitted prompt: {p!r}'
+
+    def test_with_continuation_prompt_no_turn_suffix(self) -> None:
+        runtime = PortRuntime()
+        submitted: list[str] = []
+
+        def _capture(prompt, commands, tools, denials):
+            submitted.append(prompt)
+            return _completed_result(prompt)
+
+        with patch('src.runtime.QueryEnginePort.from_workspace') as mock_factory:
+            engine = mock_factory.return_value
+            engine.submit_message.side_effect = _capture
+
+            runtime.run_turn_loop(
+                'investigate this bug',
+                max_turns=3,
+                continuation_prompt='Continue.',
+            )
+
+            # Turn 0 = original, turns 1-2 = continuation, verbatim
+            assert submitted == ['investigate this bug', 'Continue.', 'Continue.']
+            # No harness-injected suffix anywhere
+            for p in submitted:
+                assert '[turn' not in p
+                assert not p.endswith(']')
+
+
+class TestContinuationDefaultStopsAfterTurnZero:
+    def test_default_continuation_returns_one_result(self) -> None:
+        runtime = PortRuntime()
+        with patch('src.runtime.QueryEnginePort.from_workspace') as mock_factory:
+            engine = mock_factory.return_value
+            engine.submit_message.side_effect = lambda p, *_: _completed_result(p)
+
+            results = runtime.run_turn_loop('x', max_turns=5)
+            assert len(results) == 1
+            assert results[0].prompt == 'x'
+
+    def test_default_continuation_does_not_call_engine_twice(self) -> None:
+        runtime = PortRuntime()
+        with patch('src.runtime.QueryEnginePort.from_workspace') as mock_factory:
+            engine = mock_factory.return_value
+            engine.submit_message.side_effect = lambda p, *_: _completed_result(p)
+
+            runtime.run_turn_loop('x', max_turns=10)
+            # Exactly one submit_message call despite max_turns=10
+            assert engine.submit_message.call_count == 1
+
+
+class TestExplicitContinuationBehaviour:
+    def test_first_turn_always_uses_original_prompt(self) -> None:
+        runtime = PortRuntime()
+        captured: list[str] = []
+
+        def _capture(prompt, *_):
+            captured.append(prompt)
+            return _completed_result(prompt)
+
+        with patch('src.runtime.QueryEnginePort.from_workspace') as mock_factory:
+            engine = mock_factory.return_value
+            engine.submit_message.side_effect = _capture
+
+            runtime.run_turn_loop(
+                'original task', max_turns=2, continuation_prompt='keep going'
+            )
+
+            assert captured[0] == 'original task'
+            assert captured[1] == 'keep going'
+
+    def test_continuation_respects_max_turns(self) -> None:
+        runtime = PortRuntime()
+        with patch('src.runtime.QueryEnginePort.from_workspace') as mock_factory:
+            engine = mock_factory.return_value
+            engine.submit_message.side_effect = lambda p, *_: _completed_result(p)
+
+            runtime.run_turn_loop('x', max_turns=3, continuation_prompt='go')
+            assert engine.submit_message.call_count == 3
+
+
+class TestCLIContinuationFlag:
+    def test_cli_default_runs_one_turn(self) -> None:
+        """Without --continuation-prompt, CLI should emit exactly '## Turn 1'."""
+        result = subprocess.run(
+            [sys.executable, '-m', 'src.main', 'turn-loop', 'review MCP tool',
+             '--max-turns', '3', '--structured-output'],
+            check=True, capture_output=True, text=True,
+        )
+        assert '## Turn 1' in result.stdout
+        assert '## Turn 2' not in result.stdout
+        assert '[turn' not in result.stdout
+
+    def test_cli_with_continuation_runs_multiple_turns(self) -> None:
+        """With --continuation-prompt, CLI should run up to max_turns."""
+        result = subprocess.run(
+            [sys.executable, '-m', 'src.main', 'turn-loop', 'review MCP tool',
+             '--max-turns', '2', '--structured-output',
+             '--continuation-prompt', 'continue'],
+            check=True, capture_output=True, text=True,
+        )
+        assert '## Turn 1' in result.stdout
+        assert '## Turn 2' in result.stdout
+        # The continuation text is visible (it's submitted as the turn prompt)
+        # but no harness-injected [turn N] suffix
+        assert '[turn' not in result.stdout