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.

src/main.py

@@ -71,6 +71,15 @@ def build_parser() -> argparse.ArgumentParser:
         default=None,
         help='total wall-clock budget across all turns (#161). Default: unbounded.',
     )
+    loop_parser.add_argument(
+        '--continuation-prompt',
+        default=None,
+        help=(
+            'prompt to submit on turns after the first (#163). Default: None '
+            '(loop stops after turn 0). Replaces the deprecated implicit "[turn N]" '
+            'suffix that used to pollute the transcript.'
+        ),
+    )
 
     flush_parser = subparsers.add_parser('flush-transcript', help='persist and flush a temporary session transcript')
     flush_parser.add_argument('prompt')
@@ -199,6 +208,7 @@ def main(argv: list[str] | None = None) -> int:
             max_turns=args.max_turns,
             structured_output=args.structured_output,
             timeout_seconds=args.timeout_seconds,
+            continuation_prompt=args.continuation_prompt,
         )
         for idx, result in enumerate(results, start=1):
             print(f'## Turn {idx}')