Log raw LLM output preview on instruction validation failure

The CUST-WP-0045 canary failed validation twice without leaving any
record of what the model actually returned. The warning logged only the
error message ($: missing required property 'summary'), not the JSON
shape that triggered it — so diagnosing required modifying code and
re-running. Log a 2KB preview of the offending raw output alongside the
error so the next failure of this shape is one grep away from diagnosis.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

src/activity_core/rules/executor.py

@@ -156,9 +156,13 @@ def _execute(
         raw_output = llm_client.complete(retry_prompt, model=instr.model, config=llm_config)
         task_specs, report, error = _validate_output(raw_output, instr)
         if error:
+            # Truncate to keep log volume bounded but long enough to see the
+            # actual JSON shape mismatch (typical reports are <2KB).
+            preview = (raw_output or "")[:2000]
             logger.warning(
-                "instruction_output_error: instruction=%r, prompt_hash=%s, error=%s",
-                instr.id, prompt_hash, error,
+                "instruction_output_error: instruction=%r, prompt_hash=%s, "
+                "error=%s, raw_output_preview=%r",
+                instr.id, prompt_hash, error, preview,
             )
             return _empty_result(instr, prompt_hash=prompt_hash)