refactor(llm): drop the image content block until a path can honor it

ImageBlock had no production producer and every consumer dropped it:
the deepseek serializer skipped it, the pi-ai converter skipped it as
unrepresentable, the ACP bridge neither advertises image prompt
capability nor forwards image blocks, and compact-basic charged a flat
85-token estimate and rendered an [image] placeholder. A block
constructed today would silently vanish from the wire — the vocabulary
advertised a capability no path honors, the silent-data-loss shape the
defensive patterns warn against. The only constructors were tests
pinning the skip/estimate branches.

Remove ImageBlock and its ContentBlockMap entry (its cache?: CacheHint
field leaves with it; CacheHint itself and the other two cache? fields
are out of scope). compact-basic loses its explicit image estimate and
placeholder arms (the merge-extensible default arms absorb the case);
the deepseek serializer, pi-ai converter, and ACP codec already handled
image in their default arms, so only their image-naming comments
change. The codec's inbound rejection of ACP-protocol image prompt
content stays — that guards wire content a client can send regardless
of our vocabulary.

Tests that constructed harness image blocks to pin the removed branches
are dropped (the 85-token estimate pin) or retargeted onto plugin-added
block types / other non-text blocks, which the surviving default arms
own. Docs, the type-equiv pastes, and the content-block vocabulary
RFC's block list and multimodal-home consequence are updated in the
same change; the RFC moves to implemented/ and the index is
regenerated. A real multimodal feature reintroduces image via
declaration merging together with the adapter mapping, ACP
advertisement, and compaction pricing that honor it.

packages/llm/llm-deepseek/README.md

@@ -34,7 +34,6 @@ A second, independent implementation of the same seam exists in `@deepseek-ai/ds
 ## Limitations (MVP, documented deliberately)
 
 - `prefill` throws `LlmError('UNSUPPORTED')` — DeepSeek's chat-prefix completion is a Beta feature on the `/beta` base URL; future work.
-- `image` blocks are skipped (no vision support on these models).
 - `tool_choice` is not mapped (not part of the core vocabulary).
 
 ## Errors

packages/llm/llm-deepseek/src/serialize.ts

@@ -11,7 +11,6 @@
  *   rule for thinking mode — required there, ignored elsewhere, so we save
  *   the tokens elsewhere); `tool-call` → `tool_calls[]`
  * - `tool-result` → its own `{role: 'tool'}` message (text flattened)
- * - `image` → skipped (MVP limitation, documented in the README)
  *
  * @module dsh-llm-deepseek/serialize
  */

packages/llm/llm-deepseek/tests/serialize.spec.ts

@@ -1,6 +1,6 @@
 import { describe, expect, it } from 'vitest'
 import { CallId, LlmError } from '@deepseek-ai/dsh-llm'
-import type { GenerateOptions, Message } from '@deepseek-ai/dsh-llm'
+import type { ContentBlock, GenerateOptions, Message } from '@deepseek-ai/dsh-llm'
 import { serializeMessages, serializeRequest } from '@deepseek-ai/dsh-llm-deepseek'
 
 function request(overrides: Partial<GenerateOptions> = {}): GenerateOptions {
@@ -110,11 +110,17 @@ describe('serializeMessages', () => {
     ])
   })
 
-  it('skips image blocks (documented MVP limitation)', () => {
+  it('skips plugin-added block types (merge-extensible ContentBlockMap)', () => {
     const wire = serializeMessages([
-      { role: 'user', content: [{ type: 'image', url: 'data:image/png;base64,x' }, { type: 'text', text: 'see image' }] },
+      {
+        role: 'user',
+        content: [
+          { type: 'chart', data: 'x' } as unknown as ContentBlock,
+          { type: 'text', text: 'see chart' },
+        ],
+      },
     ])
-    expect(wire).toEqual([{ role: 'user', content: 'see image' }])
+    expect(wire).toEqual([{ role: 'user', content: 'see chart' }])
   })
 
   it('emits an empty user message rather than dropping block-less messages', () => {