fix(vision): replace image numbering with content-hash ids

[flobernd]

Summary

Replaces the image agent's sequential [Image #N] numbering with content-derived <image id="..."/> markers. Each stripped image's id is the first 16 hex chars of the SHA-256 of its image_url, so ids are stable across requests and history rewrites (e.g. client-side compaction), identical bytes dedupe to one cache entry, and a client's own numeric markers can never collide with a gateway id.

Intention

Clients like Claude Code write session-continuous [Image #N] markers into user text, while the strip renumbered image parts from 1 per request. Two numbering schemes of identical shape in the same turn made the model follow the client's numbers and call analyzeImage with ids that missed the per-session cache (or, in a long session, exact-hit the wrong image), burning analysis rounds. Content-hash ids remove the collision class entirely: user text passes through verbatim (no harness-specific rewriting), and a wrong id can only miss the cache, never silently resolve to the wrong image.

Changes

• The strip replaces each image with an <image id="..."/> marker carrying its content-derived id; the placeholder text, system-prompt instruction, and imageId tool-parameter description consistently instruct one batched analyzeImage call with ids copied verbatim, ignoring any other numbering in the text.
• Resolution is exact-only: VisionRequest reports unresolved_ids, and the executor injects a model-visible note listing the session's valid ids in document order so the model corrects its call in one round. The note hedges for the rare case where a correctly copied id is no longer cached (LRU-evicted or TTL-expired). Partial misses analyze the resolved images and append the correction; miss rounds still count against the analysis ceiling.
• README: the image analysis section now states the strip's actual scope - images are stripped from every user message, and residual images are file_id images or images in non-user messages.

Testing

Full cargo test green (1070 passed, 0 failed across all suites), cargo fmt --check clean, cargo clippy --all-targets zero warnings.

Notes

Stacked on local-inference-lab#36's branch; retarget or rebase once that merges.