feat(init): add native support for Kilo Code and Google Antigravity

Add rtk init --agent kilocode and rtk init --agent antigravity commands.

Kilo Code: installs .kilocode/rules/rtk-rules.md (project-scoped)
Google Antigravity: installs .agents/rules/antigravity-rtk-rules.md (project-scoped)

Both follow the same prompt-level guidance pattern as Cline and Windsurf,
using rules files that instruct the agent to prefix shell commands with rtk.