Tailing works well for append-only logs but is fundamentally racy for
files that get overwritten: fast state changes collapse into a single
truncation signal, so tail reads the wrong (already-stale) value.
Remove mutableFile, mutableVers, trackMutable, and the per-fid readBuf
entirely. Only chat retains append-only stat reporting (Qid.Vers +
Length) for tail -f. All other session files (state, usage, backend,
model, agent, cwd) are plain poll-on-read files.
Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>
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