refactor: remove unnecessary RUNTIME_ENV_PATH environment variable

Simplifies deployment configuration by removing the RUNTIME_ENV_PATH
environment variable, which is no longer needed for API mode.

Changes:
- Remove RUNTIME_ENV_PATH from docker-compose.yml
- Remove RUNTIME_ENV_PATH from .env.example
- Update CLAUDE.md to reflect API-managed runtime configs
- Update README.md to remove RUNTIME_ENV_PATH from config examples
- Update CHANGELOG.md with this simplification

Technical details:
- API mode dynamically creates isolated runtime config files via
  RuntimeConfigManager (data/runtime_env_{job_id}_{model}_{date}.json)
- tools/general_tools.py already handles missing RUNTIME_ENV_PATH
  gracefully, returning empty dict and warning on writes
- No functional impact - all tests pass without this variable set
- Reduces configuration complexity for new deployments

Breaking change: None - variable was vestigial from batch mode era

README.md

@@ -291,9 +291,6 @@ OPENAI_API_KEY=your_openai_key
 ALPHAADVANTAGE_API_KEY=your_alpha_vantage_key
 JINA_API_KEY=your_jina_api_key
 
-# ⚙️ System Configuration
-RUNTIME_ENV_PATH=./runtime_env.json # Recommended to use absolute path
-
 # 🌐 Service Port Configuration
 MATH_HTTP_PORT=8000
 SEARCH_HTTP_PORT=8001