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>

src/constants.js

@@ -93,7 +93,6 @@ export const MAX_ACCOUNTS = 10; // Maximum number of accounts allowed
 export const MAX_WAIT_BEFORE_ERROR_MS = 120000; // 2 minutes - throw error if wait exceeds this
 
 // Thinking model constants
-export const DEFAULT_THINKING_BUDGET = 16000; // Default thinking budget tokens
 export const CLAUDE_THINKING_MAX_OUTPUT_TOKENS = 64000; // Max output tokens for thinking models
 export const MIN_SIGNATURE_LENGTH = 50; // Minimum valid thinking signature length
 
@@ -131,7 +130,6 @@ export default {
     MAX_RETRIES,
     MAX_ACCOUNTS,
     MAX_WAIT_BEFORE_ERROR_MS,
-    DEFAULT_THINKING_BUDGET,
     CLAUDE_THINKING_MAX_OUTPUT_TOKENS,
     MIN_SIGNATURE_LENGTH,
     OAUTH_CONFIG,