Add resolveRunAgent() to determine agent with priority:
1. CLI --agent flag (highest)
2. OPENCODE_DEFAULT_AGENT environment variable
3. oh-my-opencode.json 'default_run_agent' config
4. 'sisyphus' (fallback)
Features:
- Case-insensitive agent name matching
- Warn and fallback when requested agent is disabled
- Pick next available core agent when default is disabled