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/go/managed-agents/README.md

@@ -65,7 +65,7 @@ fmt.Println(environment.ID) // env_...
 agent, err := client.Beta.Agents.New(ctx, anthropic.BetaAgentNewParams{
     Name: "Coding Assistant",
     Model: anthropic.BetaManagedAgentsModelConfigParams{
-        ID:   "claude-opus-4-8",
+        ID:   "claude-opus-5",
         Type: anthropic.BetaManagedAgentsModelConfigParamsTypeModelConfig,
     },
     System: anthropic.String("You are a helpful coding assistant."),
@@ -383,7 +383,7 @@ if err != nil {
 agent, err := client.Beta.Agents.New(ctx, anthropic.BetaAgentNewParams{
     Name: "GitHub Assistant",
     Model: anthropic.BetaManagedAgentsModelConfigParams{
-        ID:   "claude-opus-4-8",
+        ID:   "claude-opus-5",
         Type: anthropic.BetaManagedAgentsModelConfigParamsTypeModelConfig,
     },
     MCPServers: []anthropic.BetaManagedAgentsURLMCPServerParams{{