[backlog] Responses-only site streaming preference (upstream #340)

[DeliciousBuding]

    ## Source
    - Upstream: cita-777/metapi #340
    - Matrix: `docs/analysis/original-gap-matrix.md`
    - Category: feature-protocol
    - Status: partial · Priority: P1 · Backlog: yes
    - Epic: Protocol
    - Parent: G3 #10

    ## Evidence (from matrix)
    Responses endpoint + profile detect `proxy/profiles/codex.go`; responses-only site routing preference is partial — sites that only support `/v1/responses` + streaming need first-class handling.

    ## Acceptance criteria (draft — do not implement in G3)
    - [ ] Sites marked/detectable as responses-only route to `/v1/responses` appropriately

• Streaming preference respected for those sites

• Chat/completions clients either transform or fail clearly when site is responses-only

• Tests cover responses-only profile selection

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