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/managed-agents-tools.md

@@ -322,7 +322,7 @@ Skills are attached to the **agent** definition via `agents.create()`:
 const agent = await client.beta.agents.create(
   {
     name: "Financial Agent",
-    model: "claude-opus-4-8",
+    model: "claude-opus-5",
     system: "You are a financial analysis agent.",
     skills: [
       { type: "anthropic", skill_id: "xlsx" },
@@ -337,7 +337,7 @@ Python:
 ```python
 agent = client.beta.agents.create(
     name="Financial Agent",
-    model="claude-opus-4-8",
+    model="claude-opus-5",
     system="You are a financial analysis agent.",
     skills=[
         {"type": "anthropic", "skill_id": "xlsx"},