Two robustness gaps from the #54843 truncate-store path:
- _store_full_text wrote the full clean page to cache/web with no upper
bound (path.write_text(content)); a multi-MB page → unbounded per-extract
disk write. Cap at MAX_STORED_TEXT_CHARS (2MB, the pre-truncate-store
refusal ceiling) with a marker when capped.
- The truncation footer told the model 'read_file ... offset=<line>' — a
literal placeholder it had to guess. Compute the real starting line of the
omitted middle (head line count + 1) so the first read_file lands in the gap.
