docs: document GitHub Copilot CLI support (#30)

Copilot CLI already reads AGENTS.md and .github/copilot-instructions.md
(both shipped), plus a global ~/.copilot/copilot-instructions.md. Add an
agent-portability row and a README note. Instruction-tier only: no
/ponytail levels or hooks.

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

README.md

@@ -121,6 +121,8 @@ Cursor, Windsurf, Cline, Copilot, Aider, Kiro: copy the matching rules file from
 
 Kiro: copy `.kiro/steering/ponytail.md` to `~/.kiro/steering/` (global) or `.kiro/steering/` in your project.
 
+GitHub Copilot CLI: it already reads `AGENTS.md` and `.github/copilot-instructions.md` in a project, or copy the rules into `~/.copilot/copilot-instructions.md` to run ponytail in every project.
+
 Which files map to which agent: [Agent portability](docs/agent-portability.md).
 
 ## Development

docs/agent-portability.md

@@ -16,6 +16,7 @@ to load in a given agent.
 | Windsurf | `.windsurf/rules/ponytail.md` | Project rule. |
 | Cline | `.clinerules/ponytail.md` | Project rule. |
 | GitHub Copilot | `.github/copilot-instructions.md` | Repository instruction file. |
+| GitHub Copilot CLI | `AGENTS.md`, `.github/copilot-instructions.md`, `~/.copilot/copilot-instructions.md` | Reads custom instructions: per-project from `AGENTS.md` or `.github/copilot-instructions.md`, or globally from `~/.copilot/copilot-instructions.md`. Instruction-tier (no `/ponytail` levels or 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. |