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Supersedes the withdrawn route-scoped design of PR #598; the decision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/tool-fs/tests/tools.spec.ts

@@ -71,6 +71,13 @@ class FakeFs extends FileSystem {
     const content = this.files.get(target.targetKey) ?? ''
     return (async function* () { yield content })()
   }
+  override async readBytes(target: FsTarget, _signal: AbortSignal | undefined, maxBytes: number): Promise<Uint8Array> {
+    const bytes = new TextEncoder().encode(this.files.get(target.targetKey) ?? '')
+    if (bytes.length > maxBytes) {
+      throw new FsError(`too large: ${target.displayPath}`, 'FS_TOO_LARGE')
+    }
+    return bytes
+  }
   override async listDir(_target: FsTarget): Promise<FsDirEntry[]> {
     return []
   }