Track file mtime when read_file is called. When write_file or patch
subsequently targets the same file, compare the current mtime against
the recorded one. If they differ (external edit, concurrent agent,
user change), include a _warning in the result advising the agent to
re-read. The write still proceeds — this is a soft signal, not a
hard block.
Key design points:
- Per-task isolation: task A's reads don't affect task B's writes.
- Files never read produce no warning (not enforcing read-before-write).
- mtime naturally updates after the agent's own writes, so the warning
only fires on external changes, not the agent's own edits.
- V4A multi-file patches check all target paths.
Tests: 10 new tests covering write staleness, patch staleness,
never-read files, cross-task isolation, and the helper function.
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