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/curl/managed-agents.md

@@ -65,7 +65,7 @@ curl -X POST https://api.anthropic.com/v1/agents \
   "${HEADERS[@]}" \
   -d '{
     "name": "Coding Assistant",
-    "model": "claude-opus-4-8",
+    "model": "claude-opus-5",
     "tools": [{ "type": "agent_toolset_20260401" }]
   }'
 # → { "id": "agent_abc123", ... }
@@ -89,7 +89,7 @@ curl -X POST https://api.anthropic.com/v1/agents \
   "${HEADERS[@]}" \
   -d '{
     "name": "Code Reviewer",
-    "model": "claude-opus-4-8",
+    "model": "claude-opus-5",
     "system": "You are a senior code reviewer. Be thorough and constructive.",
     "tools": [
       { "type": "agent_toolset_20260401" },
@@ -295,7 +295,7 @@ curl -X POST https://api.anthropic.com/v1/agents \
   "${HEADERS[@]}" \
   -d '{
     "name": "MCP Agent",
-    "model": "claude-opus-4-8",
+    "model": "claude-opus-5",
     "mcp_servers": [
       { "type": "url", "name": "my-tools", "url": "https://my-mcp-server.example.com/sse" }
     ],
@@ -326,7 +326,7 @@ curl -X POST https://api.anthropic.com/v1/agents \
   "${HEADERS[@]}" \
   -d '{
     "name": "Restricted Agent",
-    "model": "claude-opus-4-8",
+    "model": "claude-opus-5",
     "tools": [
       {
         "type": "agent_toolset_20260401",