docs(claude): update Seam B summary for 0.9.0 multi-decoder routing

[lesnik512]

Summary

Closes the High finding from the 0.9.0 delta audit (planning/audit/2026-06-12-delta-audit.md): CLAUDE.md's Seam B bullet still described the pre-0.9.0 single-decoder contract — a single ResponseDecoder with only decode(content, model) — with no can_decode dispatch, no decoders=[...] list, and no MissingDecoderError pre-flight. An AI agent implementing a custom decoder from that reference ships an interface missing can_decode, which AttributeErrors when the client calls it at dispatch.

Rewritten to match the live 0.9.0 contract and stay consistent with the canonical planning/engineering.md §Seam B (which was already up to date):

• decoders: Sequence[ResponseDecoder] | None — a list, default resolves against installed extras (pydantic-first)
• dispatch walks the list; first decoder whose can_decode(model) is True runs decode(...)
• MissingDecoderError raised before the HTTP call when none claim the model
• decoder exceptions wrapped as DecodeError at the seam
• pointer to engineering.md §Seam B for the full contract

Docs-only; one line changed.

Test Plan

• No code change — CLAUDE.md only
• Matches planning/engineering.md §Seam B (lines 39–46) and the actual ResponseDecoder protocol / _dispatch_decoder behavior

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