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/java/managed-agents/README.md

@@ -57,7 +57,7 @@ import com.anthropic.models.beta.sessions.SessionCreateParams;
 // 1. Create the agent (reusable, versioned)
 var agent = client.beta().agents().create(AgentCreateParams.builder()
     .name("Coding Assistant")
-    .model("claude-opus-4-8")
+    .model("claude-opus-5")
     .system("You are a helpful coding assistant.")
     .addTool(BetaManagedAgentsAgentToolset20260401Params.builder()
         .type(BetaManagedAgentsAgentToolset20260401Params.Type.AGENT_TOOLSET_20260401)
@@ -296,7 +296,7 @@ import com.anthropic.models.beta.agents.BetaManagedAgentsUrlMcpServerParams;
 // Agent declares MCP server (no auth here — auth goes in a vault)
 var agent = client.beta().agents().create(AgentCreateParams.builder()
     .name("GitHub Assistant")
-    .model("claude-opus-4-8")
+    .model("claude-opus-5")
     .addMcpServer(BetaManagedAgentsUrlMcpServerParams.builder()
         .type(BetaManagedAgentsUrlMcpServerParams.Type.URL)
         .name("github")