fix: address ds-review-bot v7 findings on the merged image-input head

- gate model selection on steering-placement image carriers from enqueue
  until their steering/message event publishes; release the gate when an
  admission ends idle without publication (both behaviorally asserted)
- reject session.updateQueue edits carrying non-text blocks at the RPC
  boundary (queue edits cannot bypass image admission)
- extend the durable-directory walk past a first-created DSH_HOME to the
  deepest pre-existing ancestor
- strip Windows-style separators from attachment display names on POSIX
- verify attachment reads with a header-only probe (digest already proves
  the bytes decoded fully at admission); document the read path
- make SessionInputShell.addImages refusal observable and keep workspace
  transfers/composer intake from leaking refused drafts
- own ONE recursive image walk (dsh-llm contentHasImage) across apiproxy,
  pi-ai, compact-basic, and the DeepSeek text-only assertion
- drop the redundant canonical-base64 regex and the no-op role read
- move AttachmentId/AttachmentError out of types.ts (brand.ts/error.ts);
  document why AttachmentError does not extend HarnessError
- document the hard attachments inject in both consumer READMEs

packages/client/ui-conversation/tests/chat-view.spec.tsx

@@ -114,7 +114,7 @@ function makeHarness(init?: Partial<ConversationSnapshot>) {
     useInput: (() => { throw new Error('unused') }),
     inputActions: {
       setDraft: () => {},
-      addImages: () => {},
+      addImages: () => true,
       removeImage: () => {},
       pruneImages: () => {},
       submit: () => {},

packages/client/ui-conversation/tests/gate-branch-tails.spec.tsx

@@ -80,7 +80,7 @@ describe('render branch tails', () => {
         useInput={(() => { throw new Error('unused') })}
         inputActions={{
           setDraft: () => {},
-          addImages: () => {},
+          addImages: () => true,
           removeImage: () => {},
           pruneImages: () => {},
           submit: () => {},
@@ -122,7 +122,7 @@ describe('render branch tails', () => {
         useInput={(() => { throw new Error('unused') })}
         inputActions={{
           setDraft: () => {},
-          addImages: () => {},
+          addImages: () => true,
           removeImage: () => {},
           pruneImages: () => {},
           submit: () => {},

packages/client/ui-conversation/tests/input-bar.spec.tsx

@@ -339,6 +339,26 @@ describe('machine pending lock', () => {
     expect(textarea.readOnly).toBe(true)
     expect(view.container.querySelector<HTMLButtonElement>('button[aria-label="Send message"]')!.disabled).toBe(true)
   })
+
+  it('addImages reports refusal in busy phases so callers keep draft ownership', () => {
+    const { shell } = bench()
+    act(() => {
+      shell.setDraft('/goal ')
+      shell.beginCommand(
+        {
+          token: '/goal ',
+          submit: () => new Promise<never>(() => {}), // never settles: stays submitting
+        },
+        { start: 0, end: 6, draftRev: shell.snapshot.draftRev },
+      )
+      shell.submit('queue')
+    })
+    expect(shell.snapshot.phase).toBe('submitting')
+    // A refused batch must be observable (the workspace-switch transfer keeps
+    // the source shell's drafts alive instead of leaking their object URLs).
+    expect(shell.addImages(['busy-1' as never])).toBe(false)
+    expect(shell.snapshot.imageIds).toEqual([])
+  })
 })
 
 describe('decorations', () => {

packages/client/ui-conversation/tests/terminal-card.spec.tsx

@@ -395,7 +395,7 @@ describe('DetailsPanel Output section', () => {
         useSessions={bindSnapshotSelector(sessions)}
         useWorkspaces={bindSnapshotSelector(workspaces)}
         useInput={(() => { throw new Error('unused') })}
-        inputActions={{ setDraft: () => {}, addImages: () => {}, removeImage: () => {}, pruneImages: () => {}, submit: () => {} }}
+        inputActions={{ setDraft: () => {}, addImages: () => true, removeImage: () => {}, pruneImages: () => {}, submit: () => {} }}
         useProjection={(() => undefined)}
         useStore={bindSnapshotSelector(chat)}
         actions={chat.actions}
@@ -566,7 +566,7 @@ describe('DetailsPanel Output section', () => {
           baselinesReady: true, recentWorkspaceId: undefined,
         }))}
         useInput={(() => { throw new Error('unused') })}
-        inputActions={{ setDraft: () => {}, addImages: () => {}, removeImage: () => {}, pruneImages: () => {}, submit: () => {} }}
+        inputActions={{ setDraft: () => {}, addImages: () => true, removeImage: () => {}, pruneImages: () => {}, submit: () => {} }}
         useProjection={(() => undefined)}
         useStore={bindSnapshotSelector(chat)}
         actions={chat.actions}