docs: add instructions for usage with Swival (#264)

This explains how to use Ponytail with Swival.

docs/agent-portability.md

@@ -20,6 +20,7 @@ to load in a given agent.
 | GitHub Copilot CLI | `.github/plugin/`, `AGENTS.md`, `.github/copilot-instructions.md`, `~/.copilot/copilot-instructions.md` | Plugin-supported (`copilot plugin marketplace add DietrichGebert/ponytail` + `copilot plugin install ponytail@ponytail`). Fallback instruction mode remains: per-project from `AGENTS.md` or `.github/copilot-instructions.md`, or globally from `~/.copilot/copilot-instructions.md` (instruction-tier, no `/ponytail` levels or hooks). |
 | Antigravity | `AGENTS.md` | Reads `AGENTS.md` at the repo root as always-on rules (like `.cursorrules`/`CLAUDE.md`); `.agents/rules/` also works for workspace rules. Instruction-tier. |
 | CodeWhale | `AGENTS.md` | Reads `AGENTS.md` from the repo root as project instructions; also reads `CLAUDE.md` and `.claude/instructions.md` as fallbacks. Instruction-tier. |
+| Swival | `.swival/skills/`, `AGENTS.md` | `swival skills add https://github.com/DietrichGebert/ponytail` installs the six skills straight into `.swival/skills/`. Add `--global` to stage them in the library (`~/.config/swival/library`) first, then `swival skills add ponytail` (or `--global ponytail`) to activate per-project or everywhere. Also reads `AGENTS.md` from the repo root and `~/.config/swival/AGENTS.md` globally as instruction-tier fallback. |
 | VS Code + Codex extension | `AGENTS.md` | The Codex extension reads `AGENTS.md` (repo root, or `~/.codex/AGENTS.md` globally). Instruction-tier; the full Codex plugin row above adds `/ponytail` levels and hooks. |
 | Kiro | `.kiro/steering/ponytail.md` | Steering rule; copy globally or into a project. |
 | Generic agents | `AGENTS.md` or `skills/*/SKILL.md` | Copy the compact rule file or load the skill files directly. |