After a successful write_file or patch, update the stored read
timestamp to match the file's new modification time. Without this,
consecutive edits by the same task (read → write → write) would
false-warn on the second write because the stored timestamp still
reflected the original read, not the first write.
Also renames the internal tracker key from 'file_mtimes' to
'read_timestamps' for clarity.
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.
