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Supersedes the withdrawn route-scoped design of PR #598; the decision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/context/workspace-context/tests/workspace-context.spec.ts

@@ -117,6 +117,10 @@ class RecordingFileSystem extends FileSystem {
     return this.entries.get(target.targetKey)?.content ?? ''
   }
 
+  override async readBytes(_target: FsTarget, _signal: AbortSignal | undefined, _maxBytes: number): Promise<Uint8Array> {
+    throw new Error('not needed in workspace-context tests')
+  }
+
   override async streamText(target: FsTarget, signal?: AbortSignal): Promise<AsyncIterable<string>> {
     if (signal !== undefined) this.signals.push(signal)
     signal?.throwIfAborted()