[backlog] Anthropic to OpenAI skill-call anomaly (upstream #531)

[DeliciousBuding]

    ## Source
    - Upstream: cita-777/metapi #531
    - Matrix: `docs/analysis/original-gap-matrix.md`
    - Category: feature-protocol
    - Status: partial · Priority: P1 · Backlog: yes
    - Epic: Protocol
    - Parent: G3 #10

    ## Evidence (from matrix)
    Anthropic/OpenAI bridges under `transform/anthropic`, `transform/canonical`; skill-call edge cases need runtime Claude Code repro for Anthropic→OpenAI skill-call anomaly.

    ## Acceptance criteria (draft — do not implement in G3)
    - [ ] Claude Code skill-call path repro documented (pass/fail)

• Transform preserves skill-call tool semantics across Anthropic/OpenAI bridges

• Regression fixtures for skill-call multi-turn

• Known client limitations documented if not fully fixable in proxy

    ## Out of scope for this issue filing
    Docs-only backlog. No product code until a later implementation wave claims this issue.