Release v6.1.0: leaner per-session bootstrap, Codex marketplace install, Gemini removed

Bump all manifests to 6.1.0 and add RELEASE-NOTES for v6.1.0:
- Compress the using-superpowers bootstrap and prune per-harness
  tool-mapping references (lower per-session token cost).
- Add a Codex marketplace manifest so the plugin installs from Codex;
  drop the Codex SessionStart hook.
- Remove Gemini CLI support (Google EOLed the Gemini CLI 2026-06-18).

.codex-plugin/plugin.json

@@ -1,6 +1,6 @@
 {
   "name": "superpowers",
-  "version": "6.0.3",
+  "version": "6.1.0",
   "description": "An agentic skills framework & software development methodology that works: planning, TDD, debugging, and collaboration workflows.",
   "author": {
     "name": "Jesse Vincent",