Vibe and Antigravity agents missing from CommandRegistrar.AGENT_CONFIGS

Hi there,

I noticed that the `vibe` (Mistral Vibe) and `agy` (Antigravity) agents are defined in `AGENT_CONFIG` in `__init__.py` but are missing from `CommandRegistrar.AGENT_CONFIGS` in `extensions.py`.

This means extensions can't register commands for these two agents — `register_commands_for_all_agents()` will never detect them even if their directories exist.

I think this might be related to #1816 and #1705.

**Location:** `src/specify_cli/extensions.py`, `CommandRegistrar.AGENT_CONFIGS` dictionary

Happy to submit a fix if that would be helpful!