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/ruby/managed-agents/README.md

@@ -51,7 +51,7 @@ puts "Environment ID: #{environment.id}" # env_...
 # 1. Create the agent (reusable, versioned)
 agent = client.beta.agents.create(
   name: "Coding Assistant",
-  model: :"claude-opus-4-8",
+  model: :"claude-opus-5",
   system_: "You are a helpful coding assistant.",
   tools: [{type: "agent_toolset_20260401"}]
 )
@@ -265,7 +265,7 @@ client.beta.sessions.delete(session.id)
 # Agent declares MCP server (no auth here — auth goes in a vault)
 agent = client.beta.agents.create(
   name: "GitHub Assistant",
-  model: :"claude-opus-4-8",
+  model: :"claude-opus-5",
   mcp_servers: [
     {
       type: "url",