chore: bump version to 4.3.0 (#38)

Release-prep bump across the Claude Code, Codex, and Gemini manifests.
Cutting v4.3.0 also fixes #33: gemini extensions install pulls the latest
GitHub release, and gemini-extension.json was added after v4.2.0, so it
was missing from the release tarball. Shipping it in a release fixes the
plain install command.

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

gemini-extension.json

@@ -1,6 +1,6 @@
 {
   "name": "ponytail",
-  "version": "4.2.0",
+  "version": "4.3.0",
   "description": "Lazy senior dev mode. Forces the simplest, shortest solution that actually works: YAGNI, stdlib first, no unrequested abstractions.",
   "contextFileName": "AGENTS.md"
 }