Update claude-api skill: Claude Opus 5 (#1476)

* Update claude-api skill: Claude Opus 5

- Add Claude Opus 5 (`claude-opus-5`) as the default model across all
  SDK examples, model tables, pricing, and migration guidance.
- Add an Opus 5 migration section covering the API changes that come
  with it, and move Opus 4.8 into the previous-generation slot.
- Drop the Opus 4.7 fast-mode guidance, which no longer applies.
- Scope the server-side fallbacks beta header to the current version.

* Update claude-api skill: Opus 5 fast-follow corrections

- Server-side refusal fallbacks are available on Claude Platform on AWS,
  not just the Claude API. Correct the availability statements in
  SKILL.md, the migration guide, and the platform-availability table.
- Simplify the effort guidance: start at `high` (the API default) and
  sweep down, rather than starting at `xhigh` for coding work.
- Add a time-to-first-token section with the prompt instruction that
  reduces pre-answer thinking on latency-sensitive routes.
- Replace the two separate thinking-disabled mitigations with the single
  combined instruction that covers both failure modes.

skills/claude-api/shared/error-codes.md

@@ -82,7 +82,7 @@ This file documents HTTP error codes returned by the Claude API, their common ca
 - Using deprecated model ID
 - Invalid API endpoint
 
-**Fix:** Use exact model IDs from the models documentation. You can use aliases (e.g., `claude-opus-4-8`).
+**Fix:** Use exact model IDs from the models documentation. You can use aliases (e.g., `claude-opus-5`).
 
 ---
 
@@ -107,11 +107,12 @@ Some 400 errors are specifically related to parameter validation:
 - `budget_tokens` >= `max_tokens` in extended thinking
 - Invalid tool definition schema
 
-**Model-specific 400s on Fable 5 / Opus 4.8 / 4.7:**
+**Model-specific 400s on Claude Opus 5 / Fable 5 / Opus 4.8 / 4.7:**
 
 - `temperature`, `top_p`, `top_k` are removed — sending any of them returns 400. Delete the parameter; see `shared/model-migration.md` → Per-SDK Syntax Reference.
 - `thinking: {type: "enabled", budget_tokens: N}` is removed — sending it returns 400. Use `thinking: {type: "adaptive"}` instead.
-- **Fable 5 only:** an explicit `thinking: {type: "disabled"}` returns 400 (it is accepted on Opus 4.8/4.7). Omit the `thinking` param entirely instead.
+- **Claude Opus 5:** `thinking: {type: "disabled"}` returns 400 when `effort` is `xhigh` or `max` — it is accepted at `high` or below. Thinking is on by default, so omitting the param runs adaptive rather than disabling it.
+- **Fable 5 only:** an explicit `thinking: {type: "disabled"}` returns 400 at any effort (it is accepted on Opus 4.8/4.7). Omit the `thinking` param entirely instead.
 - **Fable 5 only:** if the organization is set to zero data retention (ZDR) — or any retention below the required 30 days — then **all** Fable 5 requests return `400 invalid_request_error`, even with a perfectly valid payload. Check the org's retention configuration before debugging the request body.
 
 **Common mistake with extended thinking on older models (Opus 4.6 and earlier):**
@@ -170,12 +171,12 @@ thinking: budget_tokens=10000, max_tokens=16000
 
 | Mistake                         | Error            | Fix                                                     |
 | ------------------------------- | ---------------- | ------------------------------------------------------- |
-| `temperature`/`top_p`/`top_k` on Fable 5 / Opus 4.8 / 4.7 | 400 | Remove the parameter (see `shared/model-migration.md`)  |
-| `budget_tokens` on Fable 5 / Opus 4.8 / 4.7 | 400  | Use `thinking: {type: "adaptive"}`                      |
+| `temperature`/`top_p`/`top_k` on Claude Opus 5 / Fable 5 / Opus 4.8 / 4.7 | 400 | Remove the parameter (see `shared/model-migration.md`)  |
+| `budget_tokens` on Claude Opus 5 / Fable 5 / Opus 4.8 / 4.7 | 400  | Use `thinking: {type: "adaptive"}`                      |
 | `thinking: {type: "disabled"}` on Fable 5 | 400    | Omit the `thinking` param entirely (accepted on Opus 4.8/4.7) |
 | Org set to ZDR / retention below 30 days (Fable 5) | 400 on every request | Fix the org's data-retention configuration — the payload isn't the problem |
 | `budget_tokens` >= `max_tokens` (older models) | 400 | Ensure `budget_tokens` < `max_tokens`                  |
-| Typo in model ID                | 404              | Use valid model ID like `claude-opus-4-8`               |
+| Typo in model ID                | 404              | Use valid model ID like `claude-opus-5`               |
 | First message is `assistant`    | 400              | First message must be `user`                            |
 | Consecutive same-role messages  | 400              | Alternate `user` and `assistant`                        |
 | API key in code                 | 401 (leaked key) | Use environment variable                                |