feat(fs): add a minimal read_image tool over the attachment and fs seams

The model reads a PNG/JPEG/WebP/GIF file, the bytes commit through the
durable attachment lifecycle, and the tool result carries the real
ImageBlock so the image enters context from the next request onward.
FileSystem gains a bounded readBytes primitive (local + E2B providers);
registration is conditional on the attachment store, and a strict
execution gate refuses routes that do not declare image input, so a
text route's durable history stays free of image blocks. llm-replay
models may declare inputModalities, letting keyless ACP snapshots pin
both the sha256-referenced success and the verbatim refusal.

Supersedes the withdrawn route-scoped design of PR #598; the decision
record is .agents/notes/implemented/feature/2026-08-10-minimal-read-image-tool.md.

packages/fs/fs/README.md

@@ -2,7 +2,7 @@
 
 English | [中文](README.zh.md)
 
-The **`FileSystem`** (`ctx.fs`) defines the storage primitives in one execution world — resolve paths, expose canonical process paths and file URIs, test containment, read whole or streaming text, inspect/list metadata, write atomically, and apply a literal edit — without saying HOW. Both mutations take their version guard **optionally**, so `ctx.fs` on its own is a complete, unconstrained text-storage seam. This package also owns the `fs/*` policy event vocabulary the tool dispatches and the policy plugin listens for.
+The **`FileSystem`** (`ctx.fs`) defines the storage primitives in one execution world — resolve paths, expose canonical process paths and file URIs, test containment, read whole or streaming text, read bounded raw bytes, inspect/list metadata, write atomically, and apply a literal edit — without saying HOW. Both mutations take their version guard **optionally**, so `ctx.fs` on its own is a complete, unconstrained storage seam. This package also owns the `fs/*` policy event vocabulary the tool dispatches and the policy plugin listens for.
 
 This package owns the Service Definition and provider contract layer of the four-layer filesystem stack, split so each concern can evolve (and be swapped) independently (see [the capability-seam Agent Note](../../../.agents/notes/implemented/architecture/2026-06-13-capability-seams.md), [the filesystem capability-seam Agent Note](../../../.agents/notes/implemented/architecture/2026-06-17-filesystem-capability-seam.md), [the split-the-filesystem-seam Agent Note](../../../.agents/notes/implemented/simplification/2026-06-26-fsspec-style-fs-seam.md), and [the file-context event-gate Agent Note](../../../.agents/notes/implemented/architecture/2026-06-26-file-context-as-event-gate.md)):
 
@@ -17,7 +17,7 @@ This package owns the Service Definition and provider contract layer of the four
 
 ## Service API (`ctx.fs`)
 
-A backend subclasses `FileSystem` and implements eleven primitives.
+A backend subclasses `FileSystem` and implements twelve primitives.
 
 | Member | Semantics |
 |---|---|
@@ -29,6 +29,7 @@ A backend subclasses `FileSystem` and implements eleven primitives.
 | `lstat(path, opts?, signal?)` | Return `FsPathInfo` metadata without following the final path component when it is a symlink. This is path-shaped so consumers can reject repository-owned symlinks before `resolve` follows them into a target. |
 | `readText(target, signal?)` | Read the whole regular text file as one decoded string. Owns regular-file checks, UTF-8 decoding, binary/NUL rejection (`FS_NOT_TEXT`). |
 | `streamText(target, signal?)` | Stream the same text as decoded chunks for large files (cross-chunk UTF-8 decoding stays here); consumers that need a byte ceiling enforce it while consuming the stream. |
+| `readBytes(target, signal, maxBytes)` | Read a complete regular file as raw bytes with no decoding or binary rejection. `maxBytes` is required and bounds the complete content at this seam: a known or discovered overflow fails with `FS_TOO_LARGE` instead of truncating or buffering without a bound. |
 | `listDir(target, signal?)` | List direct directory children in stable name order. Returns entry names, entry types, resolved child targets, and cheap metadata (`version`/file `size` when available); 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`. Broken/disappeared children may be returned as `other` without metadata; child permission/IO failures fail the whole listing with the same structured codes. |
 | `writeText(target, content, expected?, signal?)` | Atomic create/replace. `expected` is OPTIONAL: omit ⇒ unconditional create-or-overwrite; supply an `FsWriteIntent` (`createIfAbsent`/`replaceIfVersion`) to guard. `createIfAbsent` must perform a no-replace publication so a creator racing the initial probe is preserved. |
 | `editText(target, edit, expected?, signal?)` | Literal edit. `expected` is OPTIONAL: omit ⇒ unconditional edit of the current content; supply `{ version }` to guard (verified BEFORE matching). A missing target reports `FS_STALE_VERSION` either way. Applies and writes atomically — one mutation critical section. |
@@ -59,7 +60,7 @@ No direct invalidation; the named consumer owns any request-prefix changes.
 
 ## Known Limitations and Deferred Work
 
-- **Text-only by contract** — backends reject binary/non-UTF-8 content with `FS_NOT_TEXT`; binary-safe operations are a deliberate deferral of [the tool-schemas Agent Note](../../../.agents/notes/implemented/feature/2026-06-17-filesystem-tool-schemas.md).
-- **Eleven primitives only** — no delete, rename/move, copy, or watch; `listDir` is single-level, with recursion, globbing, pagination, and search out of scope per [the directory-listing Agent Note](../../../.agents/notes/archived/architecture/2026-07-03-filesystem-directory-listing-seam.md).
+- **Text-only mutations by contract** — text reads and both mutations reject binary/non-UTF-8 content with `FS_NOT_TEXT`; `readBytes` is the one raw-byte primitive, and binary-safe mutations remain a deliberate deferral of [the tool-schemas Agent Note](../../../.agents/notes/implemented/feature/2026-06-17-filesystem-tool-schemas.md).
+- **Twelve primitives only** — no delete, rename/move, copy, or watch; `listDir` is single-level, with recursion, globbing, pagination, and search out of scope per [the directory-listing Agent Note](../../../.agents/notes/archived/architecture/2026-07-03-filesystem-directory-listing-seam.md).
 - **No IO deadline** — the seam arms no timeout; cancellation is a best-effort optional `AbortSignal` per primitive (the deliberate [fs-family stance](../README.md)).
 - **Resolve-then-operate costs a remote backend two round-trips per tool call** — folding or caching resolution is left to such a backend.