Add GitNexus plugin

[King0James0]

Adds the GitNexus plugin: code-graph queries for Agent Zero via the GitNexus MCP server.

• Repo: https://github.com/King0James0/a0-gitnexus-plugin
• Installs the gitnexus CLI and registers gitnexus mcp as a stdio MCP server, exposing dependency/impact (blast-radius) analysis, symbol callers, route/tool maps, and Cypher queries over indexed repos.
• No API key. Cleanup-safe: uninstall removes the MCP registration and the CLI only if this plugin installed it.

Submission checklist:

• One new plugin folder (plugins/gitnexus/)
• Folder name matches name: gitnexus in the repo's root plugin.yaml
• Target repo is public with plugin.yaml and LICENSE at the root
• index.yaml within limits; tags from TAGS.md