feat: Gemini/Antigravity Skills and Extension

[dandye]

Description

This PR splits the generic setup skill into two client-specific skills:

• setup-gemini-cli: Targeted for Gemini CLI users.
• setup-antigravity: Targeted for Antigravity users, including a config template and environment variable support.

It also updates the hunt skill (extensions/google-secops/skills/hunt/SKILL.md) to:

• Remove legacy secops-mcp. and secops-soar. prefixes.
• Update tool references to match the current toolset (e.g., get_ioc_match, summarize_entity, udm_search).

Usage

Setup Antigravity Client:

1. Create extensions/google-secops/skills/setup-antigravity/.env based on .env.example.
2. Run: /security:setup_antigravity.

Setup Gemini CLI Client:

1. Run: /security:setup_gemini_cli.

Hunt Skill:

1. Run /security:hunt and follow the interactive prompts.