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-multiagent.md

@@ -13,7 +13,7 @@ The SDK sets the `managed-agents-2026-04-01` beta header automatically on all `c
 ```python
 orchestrator = client.beta.agents.create(
     name="Engineering Lead",
-    model="claude-opus-4-8",
+    model="claude-opus-5",
     system="You coordinate engineering work. Delegate code review to the reviewer and test writing to the test agent.",
     tools=[{"type": "agent_toolset_20260401"}],
     multiagent={