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Supersedes the withdrawn route-scoped design of PR #598; the decision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

@@ -96,6 +96,10 @@ class TestFileSystem extends FileSystem {
     throw new Error('not needed in skill tests')
   }
 
+  override async readBytes(_target: FsTarget, _signal: AbortSignal | undefined, _maxBytes: number): Promise<Uint8Array> {
+    throw new Error('not needed in skill tests')
+  }
+
   override async listDir(target: FsTarget): Promise<FsDirEntry[]> {
     this.listDirCalls += 1
     if (this.failListDirPaths.has(target.displayPath)) throw new Error('list temporarily failed')