fix(agents): compare rendered native agent content

[Iapa]

Summary

• compare canonical agent YAML against its rendered tool-native representation
• prevent untouched Codex TOML agents from being repeatedly reported as modified
• preserve detection of actual local TOML edits

Testing

• full test suite verified (2633 tests)
• TypeScript type check passed
• production build passed
• real CLI verification:
  • untouched Codex TOML reports no local changes
  • edited Codex TOML is detected as modified

Closes #410