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

docker-compose.yml

@@ -18,9 +18,6 @@ services:
       - ALPHAADVANTAGE_API_KEY=${ALPHAADVANTAGE_API_KEY}
       - JINA_API_KEY=${JINA_API_KEY}
 
-      # System Configuration
-      - RUNTIME_ENV_PATH=/app/data/runtime_env.json
-
       # MCP Service Ports (fixed internally)
       - MATH_HTTP_PORT=8000
       - SEARCH_HTTP_PORT=8001