feat: Centralize all prompts in prompts/ directory

New structure:
- prompts/standard_prompts.yaml: Default prompts (committed to Git)
- prompts/local/: Your improved prompts (NOT in Git!)
- prompts/README.md: Documentation
- rdagent/components/loader.py: Prompt loader with priority

Features:
- Loader checks prompts/local/ first (your better prompts)
- Falls back to standard_prompts.yaml if no local version
- Supports sections (system/user)
- Lists available prompts
- Test function included

.gitignore updated:
- prompts/local/ excluded (your proprietary prompts)
- *.local.yaml excluded
- *_private.yaml excluded

Usage:
  from rdagent.components.loader import load_prompt
  prompt = load_prompt('factor_discovery')  # Auto-loads your better version!

.gitignore

@@ -82,6 +82,11 @@ QWEN.md
 # Internal documentation (not for public)
 TODO.md
 
+# Private prompts (your improved versions)
+prompts/local/
+*.local.yaml
+*_private.yaml
+
 # Test credentials
 .env.test
 *.test.env