Add comprehensive user guide

docs/USER_GUIDE.md is a 500+ line user-facing walkthrough covering:

- What the plugin is and what it does for you
- Prerequisites + installation (both marketplace paths)
- Authentication (3 options including the credential-safe probe rationale)
- End-to-end "your first agent" walkthrough (10 steps from /lua-architect
  to /lua-deploy)
- Reference sections for all 14 slashes, 5 subagents, 9 hooks, 5 MCP tools
- 7 common workflow recipes (build/add-tool/test-prod/diagnose/rollback/
  qa-before-ship/check-deployed)
- Full safety model (deny/ask/allow rules, §3.7 single-permission
  contract, what the plugin never does)
- 13-entry troubleshooting section with copy-paste fixes
- 12-question FAQ
- Where-to-get-help links

Linked from the root README so users land on it from
github.com/lua-ai-global/claude-code-lua-plugin.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: lua-stefan-kruger

README.md

@@ -2,6 +2,8 @@
 
 A [Claude Code](https://code.claude.com) marketplace + plugin for building, testing, and deploying [Lua AI agents](https://heylua.ai) directly from inside your Claude Code session.
 
+📖 **[Read the full User Guide →](./docs/USER_GUIDE.md)** — installation, walkthroughs, slash commands reference, hooks, MCP tools, safety model, troubleshooting, FAQ.
+
 ## Install
 
 ```