The stale-write guard now refuses write_file on an existing file the task
never read in full, so test_write_file_rewrite_hint's overwrite-without-read
fixtures were refused before the hint could be computed. Reading first is
the exact read->whole-file-rewrite pattern the hint exists for.
tools-reference.md's write_file row now mirrors the WRITE_FILE_SCHEMA
description (one-sentence contract + the recovery step) instead of a
longer paraphrase.
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