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).

.claude-plugin/marketplace.json

@@ -9,7 +9,7 @@
     {
       "name": "superpowers",
       "description": "Core skills library for Claude Code: TDD, debugging, collaboration patterns, and proven techniques",
-      "version": "6.0.3",
+      "version": "6.1.0",
       "source": "./",
       "author": {
         "name": "Jesse Vincent",

.claude-plugin/plugin.json

@@ -1,7 +1,7 @@
 {
   "name": "superpowers",
   "description": "Core skills library for Claude Code: TDD, debugging, collaboration patterns, and proven techniques",
-  "version": "6.0.3",
+  "version": "6.1.0",
   "author": {
     "name": "Jesse Vincent",
     "email": "jesse@fsck.com"