docs: host lists name Copilot, Antigravity and Gemini CLI

Co-authored-by: capy-ai[bot] <230910855+capy-ai[bot]@users.noreply.github.com>

docs/PROJECT_STRUCTURE.md

@@ -20,6 +20,7 @@ gstack/
 │   ├── codex.ts, factory.ts, kiro.ts  # Existing hosts
 │   ├── opencode.ts, slate.ts, cursor.ts, openclaw.ts  # IDE hosts
 │   ├── hermes.ts, gbrain.ts  # Agent runtime hosts
+│   ├── copilot.ts, agy.ts, gemini.ts  # GitHub Copilot CLI, Antigravity CLI, Gemini CLI
 │   └── index.ts     # Registry: exports all, derives Host type
 ├── scripts/         # Build + DX tooling
 │   ├── gen-skill-docs.ts  # Template → SKILL.md generator (config-driven)