feat(plugins): scaffold git, gitea, and core plugins

Adds three new plugins to the holocron marketplace:
- git: 4 stub skills (git-commit, git-branch, git-pr, git-flow)
- gitea: 5 stub skills (gitea-issues, gitea-prs, gitea-milestones, gitea-releases, gitea-wiki)
- core: 5 stub skills (triage, diagnose, zoom-out, caveman, improve-codebase-architecture)

All plugins follow version-parity convention (ADR-0016). Skills are
placeholder stubs — content will be authored in a future workstream.
Registered in both marketplace.json paths.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

plugins/git/.claude-plugin/plugin.json

@@ -0,0 +1,17 @@
+{
+  "author": {
+    "name": "Defame1297",
+    "url": "https://git.dev.rkdr.net/Defame1297/"
+  },
+  "description": "Skills for working with Git \u2014 conventional commits, branch management, pull requests, and feature flow.",
+  "displayName": "Git",
+  "keywords": [
+    "git",
+    "vcs",
+    "commit",
+    "branch"
+  ],
+  "license": "MIT",
+  "name": "git",
+  "version": "1.0.0"
+}