fix(extraction): R11 container 503 + R12 shared prompt module

R11: POST /admin/extract-batch with mode=llm now returns 503 when the
claude CLI is unavailable (was silently returning success with 0
candidates), with a message pointing at the host-side script. +2 tests.

R12: extracted SYSTEM_PROMPT + parse_llm_json_array +
normalize_candidate_item + build_user_message into stdlib-only
src/atocore/memory/_llm_prompt.py. Both the container extractor and
scripts/batch_llm_extract_live.py now import from it, eliminating the
prompt/parser drift risk.

Tests 297 -> 299.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

src/atocore/api/routes.py

@@ -55,6 +55,7 @@ from atocore.memory.extractor import (
 )
 from atocore.memory.extractor_llm import (
     LLM_EXTRACTOR_VERSION,
+    _cli_available as _llm_cli_available,
     extract_candidates_llm,
 )
 from atocore.memory.reinforcement import reinforce_from_interaction
@@ -832,6 +833,18 @@ def api_extract_batch(req: ExtractBatchRequest | None = None) -> dict:
     invoke this endpoint explicitly (cron, manual curl, CLI).
     """
     payload = req or ExtractBatchRequest()
+
+    if payload.mode == "llm" and not _llm_cli_available():
+        raise HTTPException(
+            status_code=503,
+            detail=(
+                "LLM extraction unavailable in this runtime: the `claude` CLI "
+                "is not on PATH. Run host-side via "
+                "`scripts/batch_llm_extract_live.py` instead, or call this "
+                "endpoint with mode=\"rule\"."
+            ),
+        )
+
     since = payload.since
 
     if not since: