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.

docs/rfc/implemented/simplification/2026-07-04-drop-image-content-block.md

@@ -0,0 +1,27 @@
+# RFC: Drop the `image` content block until a path can honor it
+
+Status: implemented (proposed and accepted 2026-07-04)
+
+## Problem
+
+`ImageBlock` (`packages/llm/llm/src/types.ts`) had no production producer, and every consumer on every path DROPPED it: the deepseek adapter's serializer skipped image blocks (a documented MVP limitation), the pi-ai converter skipped them as unrepresentable, the ACP codec neither advertises image prompt capability nor forwarded image blocks outbound and REJECTS image prompt content inbound, and the compaction estimator charged a flat token constant and rendered `[image]`. An `ImageBlock` constructed then would silently vanish from the wire — the vocabulary advertised a capability no path honored, which is the silent-data-loss shape AGENTS.md's defensive patterns warn against. The only constructors anywhere were tests pinning the skip/drop/estimate branches.
+
+## Decision
+
+Remove `ImageBlock`, its `ContentBlockMap` entry (and its `cache?: CacheHint` field with it), the explicit `image` estimate/placeholder arms in compact-basic, and the image-naming comments in the deepseek serializer's, pi-ai converter's, and ACP codec's default arms — those default arms absorb the case the way they absorb any unknown block type. Updated in the same change: the vocabulary line in [architecture.md](../../../architecture.md), the block list in `packages/llm/llm/README.md`, the deepseek README's image-skip row, the pi-ai README's images-not-representable row, the compact-basic README's image-estimation and `[image]`-placeholder rows, the pastes in [core.md](../../../core-data-structures/core.md) and [llm-streaming.md](../../../core-data-structures/llm-streaming.md), and the [content-block vocabulary RFC](../architecture/2026-06-11-content-block-vocabulary.md)'s block list and multimodal-home consequence per [implemented/AGENTS.md](../AGENTS.md); the tests that constructed image blocks to exercise the removed branches were dropped (the estimate pin) or retargeted onto the merge-extensible default arms (plugin-added block types). The ACP codec's inbound rejection of image PROMPT content is unaffected — that guard is about protocol content a client can send regardless of our vocabulary, and it stays.
+
+## Why not keep it?
+
+This was the most contested cut in the batch. Multimodal input (screenshots) is a plausible near-term coding-agent feature, and the [content-block vocabulary RFC](../architecture/2026-06-11-content-block-vocabulary.md) reserved the slot deliberately. Two responses. First, `ContentBlockMap` is merge-extensible by design: a real multimodal feature reintroduces `image` in core in the same coordinated change that maps it in the adapters, advertises and renders it in ACP, and prices it in compaction — the producer and its consumers arrive together, which is how the map is meant to grow. Second, the middle option — keep the type but make adapters throw UNSUPPORTED instead of silently dropping — converts this into exactly the shape the sibling request-knobs proposal (`2026-07-04-drop-inert-request-knobs`) argues against: surface whose only implementation is rejection. Absence (a compile error at the would-be producer) is strictly clearer than either silent loss or universal throw.
+
+The recorded fallback, had review landed on keeping the slot: keep `ImageBlock` but replace every silent skip with a loud rejection, and document that policy in the vocabulary — the silent drop was the one state with no defender. Review landed on removal; the fallback stands as the documented alternative should the slot ever return ahead of a full feature.
+
+## Acceptance criteria
+
+- No `ImageBlock` / harness `type: 'image'` block construction outside this RFC; the codec's inbound ACP-image rejection still passes its tests.
+- Adapter/codec/compaction switches handle the case through their unknown-block default arms (pinned by the plugin-added-block tests).
+- Doc pastes, the manifest, and the architecture vocabulary list updated; `pnpm run doc-sync` green.
+
+## Risks
+
+Re-adding a core vocabulary type later touches several packages at once — but that coordinated change is the shape a real multimodal feature needs anyway (adapter mapping, ACP advertisement, compaction pricing), and none of it existed to preserve.