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/compact/compact-basic/src/index.ts

@@ -44,9 +44,6 @@ export { resolveConfig } from './types.ts'
 /** Per-block structural overhead for JSON framing / type tag. */
 const BLOCK_OVERHEAD = 4
 
-/** Heuristic token count for an image block (~85 tokens for low-res URL). */
-const IMAGE_TOKEN_COST = 85
-
 /** Role-field framing overhead added per message in {@link BasicCompactService.estimateTokens}. */
 const ROLE_OVERHEAD = 4
 
@@ -230,9 +227,6 @@ export class BasicCompactService extends CompactService {
         case 'tool-result':
           tokens += this.estimateContentTokens(block.content) + BLOCK_OVERHEAD
           break
-        case 'image':
-          tokens += IMAGE_TOKEN_COST
-          break
         default:
           // Unknown block types (merge-extensible ContentBlockMap):
           // estimate conservatively via JSON stringify.
@@ -706,10 +700,10 @@ export class BasicCompactService extends CompactService {
   /**
    * Render content blocks to a single plain-text string for the summarization
    * prompt. Text and reasoning contribute their text; every other block type
-   * contributes a type-tagged placeholder (`[image]`, `[tool-call: name(args)]`,
-   * …) so the summarizer is told what non-text content existed in the region
-   * rather than silently losing it. Blocks join with newlines; empty-text
-   * blocks contribute nothing.
+   * contributes a type-tagged placeholder (`[tool-call: name(args)]`,
+   * `[tool-result: …]`, …) so the summarizer is told what non-text content
+   * existed in the region rather than silently losing it. Blocks join with
+   * newlines; empty-text blocks contribute nothing.
    */
   private _blocksToText(blocks: readonly ContentBlock[]): string {
     const parts: string[] = []
@@ -729,9 +723,6 @@ export class BasicCompactService extends CompactService {
           parts.push(inner ? `[tool-result: ${inner}]` : '[tool-result]')
           break
         }
-        case 'image':
-          parts.push('[image]')
-          break
         // ContentBlockMap is merge-extensible — render an unknown block as a
         // bare type-tagged placeholder so a plugin-added block type is still
         // signalled to the summarizer rather than dropped.