Agent Note: Prune write-only fields and a dead routing knob from the fs seam

Status: implemented

Problem

The fs seam split moved read routing and policy out of the backend into dsh-tool-fs and dsh-fs-policy. Four pieces of surface kept the pre-split shape — populated on every call, read by nobody:

STREAM_MIN_SIZE + FsIoInternals.streamMinSize in dsh-fs-local — removed ahead of this change by the no-hardcoded-tunables audit, which made the routing bound dsh-tool-fs's readStreamMinSize config; recorded here as part of the full prune. Originally (packages/fs/fs-local/src/fsio.ts, re-exported from packages/fs/fs-local/src/index.ts): zero readers anywhere, including fs-local's own source and tests. The backend has no read routing — readWholeText/streamWholeText are separate primitives the caller chooses between — and the real routing constant lives in the consumer (packages/fs/tool-fs/src/read.ts, compared against info.size). Two mirrors of the 10 MiB fact; the backend's was dead, and the knob's JSDoc claimed a "read routing" override that did not exist.
FsTarget.inputPath (packages/fs/fs/src/types.ts): every backend and every test fake had to fabricate a "diagnostics only" value with zero production readers — the policy plugin and every error message use targetKey/displayPath. The listDir producer exposed the semantic wobble: directory children got the bare entry name, which was nobody's "input".
FsEditOutcome.replacements + .replaceAll (packages/fs/fs/src/types.ts): replacements had zero production readers (the single-match policy itself stays — it is enforced by the FS_AMBIGUOUS_EDIT/FS_EDIT_NOT_FOUND throws inside the backend, whose error message keeps the internal count); replaceAll was read only by formatEditOutput in packages/fs/tool-fs/src/edit.ts — as an echo of the replace_all argument the tool already holds. Shrunk, FsEditOutcome is { version, before, after }, parallel to FsWriteOutcome's genuinely backend-discovered fields.
FileReadOutcome.limit + .version (packages/fs/tool-fs/src/read-render.ts): populated by the read tool, but formatReadOutput renders offset/lines/totalLines/truncatedByBytes only, and the fs/observed emit uses info.version directly rather than an outcome copy.

Decision

Delete the fs-local constant, its re-export, and the streamMinSize knob (the remaining FsIoInternals knobs are genuinely used by the atomic-write tests); drop inputPath from FsTarget; shrink FsEditOutcome to { version, before, after } and pass replaceAll to formatEditOutput from the parsed args; drop limit/version from FileReadOutcome. The filesystem.md pastes, packages/fs/fs/README.md, and the test fakes that had to fabricate the removed fields shrink with the types.

Alternatives considered

Why not keep them?

A future permission/containment layer might want the pre-resolution path for error text — but it would want the request, which every call site still holds. "N occurrences replaced" might become model-facing text — a behavior change to design when wanted, and the backend-internal count survives for its error message. A read footer might display limit — everything the footer shows already derives from lines/totalLines. Meanwhile every current and future backend (remote, native) would have to fabricate wire fields nobody consumes, and every test fake would have to satisfy them.

Verification

The removed surfaces are gone — STREAM_MIN_SIZE/streamMinSize in dsh-fs-local, FsTarget.inputPath, FsEditOutcome.replacements/.replaceAll, and FileReadOutcome.limit/.version — while the request-side replaceAll (FsEditRequest) and the version fields on the other outcome types are untouched; the test fakes shrank with the types. formatEditOutput's emitted text is unchanged for both replace_all branches, so no snapshot expected output churned.

Consequences

Backends gain no new obligations; they shed four fields nobody consumed. The fs discovery work (glob/grep tools) touches the same dsh-fs type files — a textual, not design, overlap that reconciles mechanically.

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