Abstract filesystem provider service. Subclass, implement the seven storage primitives, and load the subclass as a plugin — it registers as `ctx.fs` (one implementation per context; loading a second throws, cordis' standard duplicate-service behavior).
Semantics every backend must honor:
- resolve returns a stable FsTarget; the same underlying file reached by different input paths must yield the same `targetKey` so stale guards and target lookup agree across paths (e.g. through symlinks).
- stat returns FsInfo metadata (never content) or `undefined` when the target is absent.
- readText/streamText read the whole regular text file (the stream for large files); both own regular-file checks, UTF-8 decoding, binary/NUL rejection, and `FS_NOT_TEXT`.
- listDir returns direct children of a directory in stable name order with resolved child targets and cheap metadata only. It never reads file contents. Missing targets throw `FS_NOT_FOUND`, non-directories throw `FS_NOT_DIRECTORY`, permission failures throw `FS_PERMISSION_DENIED`, and other backend I/O failures throw `FS_IO_ERROR`.
- writeText is atomic temp-file + rename. `expected` is OPTIONAL: omit it for an unconditional create-or-overwrite (the bare-provider default), or supply a FsWriteIntent to guard the write.
- editText verifies `expected.version` BEFORE literal matching (so a stale edit reports `FS_STALE_VERSION`, not `FS_EDIT_NOT_FOUND`/ `FS_AMBIGUOUS_EDIT` against newer content), then applies literal replacement and writes atomically — all inside one mutation critical section. `expected` is OPTIONAL: omit it for an unconditional edit of the current content (a missing target still reports `FS_STALE_VERSION`).
Resolve a model/plugin-supplied path into a stable FsTarget. May perform I/O (a remote/sandboxed backend may need a round-trip to map a path to a stable identity), hence async even though the local backend only normalizes + realpaths.
`opts.cwd` is the base directory a RELATIVE `path` resolves against; an absolute `path` ignores it. Omitted ⇒ the backend's own default base (the local backend uses its configured `cwd`). The CALLER supplies this — the seam does not read a session or agent — so a tool can resolve against the caller's per-session workspace (`exec.agent.session.header.cwd`) without the provider depending on `dsh-agent`/`dsh-session`. Mirrors how `dsh-tool-bash` defaults a bash `workdir` to the session cwd.
- `path` — the path to resolve; relative paths resolve against `opts.cwd`.
- `opts` — `cwd` overrides the backend's default base for relative paths.
**Returns** the stable target; the same file yields the same `targetKey`.
Stream the whole regular text file as decoded text chunks (same text semantics as readText, for large files). The backend owns cross-chunk UTF-8 decoding and binary rejection so the policy layer never touches raw bytes.
Create or fully replace a UTF-8 text file atomically. `expected` is the create-vs-replace decision and stale guard when supplied; OMITTING it is an unconditional create-or-overwrite (the bare provider — no version guard, no read-first requirement). Atomic either way.
- `target` — the resolved target to write.
- `content` — the full new file content.
- `expected` — the write intent guarding the write; omit for unconditional.
- `signal` — aborts before the atomic rename takes effect.
**Returns** the outcome, including the version the write produced.
Apply a literal edit to an existing UTF-8 text file. When `expected` is supplied, verifies `expected.version` as the stale guard BEFORE literal matching; OMITTING it edits the current content unconditionally (no version guard). Either way applies the replacement and writes atomically — one mutation critical section — and a missing target reports `FS_STALE_VERSION`.
