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/llm/llm-pi-ai/tests/adapter.spec.ts

@@ -638,7 +638,7 @@ describe('provider profile lifecycle', () => {
 describe('abort wiring', () => {
   it('preserves an unknown pre-dispatch adapter Error exactly', async () => {
     const original = new Error('SDK context conversion exploded')
-    const message = Object.defineProperty({}, 'role', {
+    const message = Object.defineProperty({}, 'content', {
       get() { throw original },
     })
     const adapter = new PiAiAdapter({ profiles: [{ provider: 'deepseek', apiKey: 'test-key' }] })
@@ -656,7 +656,7 @@ describe('abort wiring', () => {
   it('lets a concurrent caller abort classify a pre-dispatch adapter failure', async () => {
     const controller = new AbortController()
     const original = new Error('conversion lost its caller')
-    const message = Object.defineProperty({}, 'role', {
+    const message = Object.defineProperty({}, 'content', {
       get() {
         controller.abort('caller cancelled during conversion')
         throw original