docs: document Antigravity and VS Code Codex extension support (#36)

Both read AGENTS.md, which the repo already ships, so ponytail works
from the repo root with no extra setup. Add agent-portability rows and a
README note. Instruction-tier (no /ponytail levels or hooks).

Co-authored-by: Claude Opus 4.8 (1M context) <noreply@anthropic.com>