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/php/managed-agents/README.md

@@ -48,7 +48,7 @@ use Anthropic\Beta\Agents\BetaManagedAgentsAgentToolset20260401Params;
 // 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: [
         BetaManagedAgentsAgentToolset20260401Params::with(
@@ -307,7 +307,7 @@ use Anthropic\Beta\Sessions\BetaManagedAgentsAgentParams;
 // 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',
     mcpServers: [
         BetaManagedAgentsURLMCPServerParams::with(
             type: 'url',