feat: add prompt caching, sticky account selection, and non-thinking model

- Implement sticky account selection for prompt cache continuity
- Derive stable session ID from first user message (SHA256 hash)
- Return cache_read_input_tokens in usage metadata
- Add claude-sonnet-4-5 model without thinking
- Remove DEFAULT_THINKING_BUDGET (let API use its default)
- Add prompt caching test
- Update README and CLAUDE.md documentation

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

package.json

@@ -17,7 +17,8 @@
     "test:multiturn": "node tests/test-multiturn-thinking-tools.cjs",
     "test:streaming": "node tests/test-multiturn-thinking-tools-streaming.cjs",
     "test:interleaved": "node tests/test-interleaved-thinking.cjs",
-    "test:images": "node tests/test-images.cjs"
+    "test:images": "node tests/test-images.cjs",
+    "test:caching": "node tests/test-caching-streaming.cjs"
   },
   "keywords": [
     "claude",