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

.env.example

@@ -13,9 +13,6 @@ OPENAI_API_KEY=your_openai_key_here # https://platform.openai.com/api-keys
 ALPHAADVANTAGE_API_KEY=your_alphavantage_key_here  # https://www.alphavantage.co/support/#api-key
 JINA_API_KEY=your_jina_key_here # https://jina.ai/
 
-# System Configuration (Docker default paths)
-RUNTIME_ENV_PATH=/app/data/runtime_env.json
-
 # MCP Service Host Ports (exposed on host machine)
 # Container always uses 8000-8003 internally
 # Change these if you need different ports on your host