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

CHANGELOG.md

@@ -7,6 +7,13 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 ## [Unreleased]
 
+### Changed
+- **Simplified Configuration** - Removed `RUNTIME_ENV_PATH` environment variable
+  - No longer needed in docker-compose.yml or .env.example
+  - API mode dynamically manages runtime configs via `RuntimeConfigManager`
+  - Legacy tools gracefully handle missing `RUNTIME_ENV_PATH`
+  - Reduces configuration complexity for new deployments
+
 ## [0.3.0] - 2025-10-31
 
 ### Added - API Service Transformation