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/shared/token-counting.md

@@ -16,7 +16,7 @@ from anthropic import Anthropic
 
 client = Anthropic()
 resp = client.messages.count_tokens(
-    model="claude-opus-4-8",
+    model="claude-opus-5",
     messages=[{"role": "user", "content": open("CLAUDE.md").read()}],
 )
 print(resp.input_tokens)
@@ -28,7 +28,7 @@ TypeScript: `await client.messages.countTokens({model, messages})` →
 ## CLI
 
 ```sh
-ant messages count-tokens --model claude-opus-4-8 \
+ant messages count-tokens --model claude-opus-5 \
   --message '{role: user, content: "@./CLAUDE.md"}' \
   --transform input_tokens -r
 ```
@@ -44,7 +44,7 @@ import subprocess
 client = Anthropic()
 def count(text: str) -> int:
     return client.messages.count_tokens(
-        model="claude-opus-4-8",
+        model="claude-opus-5",
         messages=[{"role": "user", "content": text}],
     ).input_tokens