A memory-only tools[] pin self-heals: restore_agent_tool_prefix appends every
fresh tool to the pin and persists it (merged != pinned) on the next turn. The
clear_pin flag was also unreachable in scan mode (findings need skill_manage).
So drop clear_pin/_HYGIENE_PIN_TOOLS and clear_system_prompt_for_rebuild, and
reuse update_system_prompt(sid, None), which already nulls prompt+hash and GCs
in one write.
The detector now parses each pin and checks the marker once per row, keyed on
prompt_builder.SKILL_SAFETY_HEADING instead of a hand-copied literal. The scan
classifies each compact_rows page as it arrives, keeps only finding dicts, and
dedupes ids that OFFSET paging can re-serve during concurrent inserts.
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