fix: merge agent command with args in parseAgents config

- Updated parseAgents() in src/config.ts to read raw.args and merge
  it with command as "{command} {args.join(' ')}"
- Updated ConfigAgentEntry type to include args?: string[]
- Added 3 tests: args merging, backward compat, invalid type validation
- All 443 tests pass, lint clean

Fixes #198

Author: log-li