feat: add Gemini CLI support (#25)

* feat: add Gemini CLI support

Add a thin Gemini CLI extension adapter (closes #22). The manifest points
contextFileName at the existing AGENTS.md for always-on rules and reuses the
repo's commands/*.toml (/ponytail, /ponytail-review) and skills/, which Gemini
CLI auto-discovers — no rule text is duplicated.

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

* test: read manifest inside cases for a clean RED

The manifest was parsed at module scope, so reverting the adapter crashed the
whole test file at load (ENOENT) and collapsed all four cases into one
unreadable failure. Read it inside each case via a helper that asserts existence
first, so a missing or malformed manifest surfaces as clear per-case assertion
failures instead of a stack trace.

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

---------

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

README.md

@@ -11,7 +11,7 @@
 <p align="center">
   <img src="https://img.shields.io/github/stars/DietrichGebert/ponytail?style=flat-square&color=111111&label=stars" alt="Stars">
   <img src="https://img.shields.io/github/v/release/DietrichGebert/ponytail?style=flat-square&color=111111&label=release" alt="Release">
-  <img src="https://img.shields.io/badge/works%20with-10%20agents-111111?style=flat-square" alt="Works with 10 agents">
+  <img src="https://img.shields.io/badge/works%20with-11%20agents-111111?style=flat-square" alt="Works with 11 agents">
   <img src="https://img.shields.io/badge/license-MIT-111111?style=flat-square" alt="MIT license">
 </p>
 
@@ -101,6 +101,14 @@ Run OpenCode from a checkout of this repo (the plugin reuses its `hooks/` and `s
 
 Injects the ruleset every turn at the active level; adds `/ponytail`, `/ponytail-review`, and `/ponytail-audit`. OpenCode also auto-loads this repo's `AGENTS.md`, so the rules hold even without the plugin. The plugin adds the `lite/full/ultra/off` levels.
 
+### Gemini CLI
+
+```bash
+gemini extensions install https://github.com/DietrichGebert/ponytail
+```
+
+Loads the ruleset as always-on context every session and registers `/ponytail` and `/ponytail-review`; the `skills/` ship too, activated when a task needs them.
+
 That was it. He'd be proud. He won't say it.
 
 Active every session. `/ponytail-review` finds what to delete in your diff, `/ponytail-audit` does the same for the whole repo. `/ponytail ultra` exists for when the codebase has wronged you personally. `/ponytail-help` explains the rest.