Steer skills off /call to compat endpoints, add /call→compat migration

[Jose-Sabater]

We're sunsetting `/call` (both `POST /v3/call` and the legacy v2 Python SDK's `opper.call`). Best practice for structured one-shot tasks is now a compat chat endpoint with `response_format`. This PR makes the skills reflect that everywhere and gives existing `/call` users a concrete migration path.

What changed

`opper-api/references/migration.md` — new opening section: From Opper's own `/call`, with verified before/after curls (both run against prod), a field-by-field mapping table (`name` → `X-Opper-Name` header, `instructions` → system message, `output_schema` → `response_format: json_schema`, `data` → parsed `choices[0].message.content`, `meta.cost` → `usage.opper.cost.total` / `X-Opper-Cost`), and a Python before/after using the stock OpenAI SDK's `chat.completions.parse()` with a Pydantic model. Retitled since it now covers more than external gateways.

`opper-api/SKILL.md` — "Still on /call?" bullet in Pick your endpoint, a sunset gotcha, migration section + description mention so the skill triggers on "migrate off opper.call".

`opper-sdks/SKILL.md` — no longer recommends `opper.call` for one-shot tasks. Compat + `response_format` is the recommended primitive; canonical seeds are now OpenAI-SDK-against-compat (Python + TS) with `X-Opper-Name`; `opper.call`/`opper.stream` explicitly marked legacy with a migration pointer. Agent SDK and `opper.knowledge.*` unchanged. Legacy banners added to `references/python.md` / `references/typescript.md`.

`opper/SKILL.md` (router) — first-call guidance for new users now lands on compat (CLI still fronts auth/usage/traces/launch); `opper call` removed from proposals and Phase-4 verification; follow-up no longer suggests swapping the provider SDK for `opperai`.

`opper-cli/SKILL.md` — `opper call` marked legacy in the command table + a gotcha: fine for an ad-hoc poke, never the pattern to copy into application code.

Verification

• Both migration-doc curls executed verbatim against prod: `/v3/call` and the compat equivalent return the same structured object; `X-Opper-Name` + `meta.trace_uuid` confirmed on the compat side.
• `scripts/validate-skills.sh` passes.

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