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>

gemini-extension.json

@@ -0,0 +1,6 @@
+{
+  "name": "ponytail",
+  "version": "4.2.0",
+  "description": "Lazy senior dev mode. Forces the simplest, shortest solution that actually works: YAGNI, stdlib first, no unrequested abstractions.",
+  "contextFileName": "AGENTS.md"
+}