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>

.claude-plugin/marketplace.json

@@ -15,6 +15,21 @@
       "description": "A place for things to be binned",
       "name": "bin",
       "source": "./plugins/bin"
+    },
+    {
+      "description": "Skills for working with Git \u2014 conventional commits, branch management, pull requests, and feature flow.",
+      "name": "git",
+      "source": "./plugins/git"
+    },
+    {
+      "description": "Skills for managing Gitea repositories \u2014 issues, pull requests, milestones, releases, and wikis.",
+      "name": "gitea",
+      "source": "./plugins/gitea"
+    },
+    {
+      "description": "Cross-cutting utility skills for everyday AI-assisted coding \u2014 triage, diagnosis, architecture review, and session navigation.",
+      "name": "core",
+      "source": "./plugins/core"
     }
   ],
   "version": "0.1.1"