feat(agents): wire agent_definitions and opencode.json agents into precedence chain

- Modified agent-config-handler.ts to load and integrate both new agent sources
- Added loadAgentDefinitions() and readOpencodeConfigAgents() calls in loading phase
- Integrated both sources into agent precedence chains (both Sisyphus-enabled and disabled paths)
- Added detailed logging for new agent sources
- Added filtering logic to respect disabled_agents configuration
- Extended agent-config-handler.test.ts with 7 new integration tests
- All tests passing (18/18 integration, 65/65 loader suite)

Wave 3 of agent definitions enhancement complete.