feat(agents): add agent definitions file loader and opencode.json reader

- Add loadAgentDefinitions() for explicit file path loading (.md/.json/.jsonc)
- Add readOpencodeConfigAgents() for independent opencode.json(c) reading
- Extract parseMarkdownAgentFile() from loader.ts for reuse
- Refactor loader.ts to use extracted parser (-50 LOC)
- Add comprehensive test coverage (13 tests for definitions loader, 10 tests for opencode reader)
- Support inline agents + agent_definitions paths in opencode.json(c)
- Inline agents override definition-file agents (correct precedence)

Part of agent definitions enhancement (Wave 2/3)