Two cohesion cleanups on the filesystem tool package:
- Fold window.ts + types.ts + formatReadOutput into one cordis-free
read-render.ts. Line windowing, the FileReadOutcome shape, and output
formatting are one concern (the read tool's rendering); splitting them across
three files added no value. read.ts is now just the tool (schema + I/O).
- Drop observe.ts and emit fs/observed with a plain ctx.emit in read/write/edit.
The event is contractually a synchronous, side-effect-only recorder
(file-context's listener is a WeakMap.set), so the per-call try/catch guarded
against a contract violation that cannot happen under the shipped listener —
defensive code for an impossible case. The event contract (dsh-fs JSDoc,
README, RFC) is updated to state the fire-and-forget semantics plainly.
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