fix(gui): address ds-review-bot findings on image attachments

- dsh web gains --provider/--model: a non-deepseek provider mounts the
  matching pi-ai catalog route (ambient credentials), making image input
  reachable from the shipped Web assembly; requires an explicit --model
- attachment-local syncs the publication directories after the hard-link
  publish so a reported durable reference survives a crash (POSIX; Windows
  relies on filesystem metadata journaling)
- the attachment seam gains storage-free validateImage; the host validates a
  complete multi-image prompt before persisting any member, so one malformed
  image cannot strand valid members as unreferenced objects
- startSession sends before navigating: a rejected first send keeps the empty
  state, its error strip, and the complete draft mounted
- the webserver rejects an undeclared-length body the moment it crosses the
  configured limit instead of draining a potentially endless stream to EOF

packages/attachment/attachment/README.md

@@ -2,7 +2,7 @@
 
 The durable attachment seam. `ctx.attachments` validates and atomically commits immutable image bytes, then returns a serializable `ImageAttachmentRef`; consumers never persist browser paths, object URLs, provider URLs, or base64 in session events.
 
-Unsent composer images remain browser-owned temporary drafts. `saveImage` is called only at message submission or while committing structured provider output, before any model-visible session event is published. `readImage` verifies the content-addressed object against its logged metadata.
+Unsent composer images remain browser-owned temporary drafts. `saveImage` is called only at message submission or while committing structured provider output, before any model-visible session event is published. `validateImage` runs the same admission policy without persisting; batch writers validate every member first so one malformed member cannot strand earlier members as unreferenced objects (there is no garbage collection). `readImage` verifies the content-addressed object against its logged metadata.
 
 ## Model Experience
 

packages/attachment/attachment/src/index.ts

@@ -33,6 +33,14 @@ export abstract class AttachmentStore extends Service {
   /** Deployment-resolved image policy used by authoritative and fast-path validation. */
   abstract readonly imageLimits: ImageAttachmentLimits
 
+  /**
+   * Validate one image against the deployment policy without persisting anything.
+   * Callers persisting a multi-image batch validate every member first so a
+   * malformed member cannot leave earlier members as unreferenced objects.
+   * @param input - encoded bytes, declared media type, and optional display name.
+   */
+  abstract validateImage(input: SaveImageAttachment): void
+
   /**
    * Validate and durably commit one image before its owning session event is appended.
    * @param input - encoded bytes, declared media type, and optional display name.