docs: create USER-GUIDE.md with configuration and advanced usage

[snipcodeit]

Summary

• Created comprehensive docs/USER-GUIDE.md covering the full MGW user experience from installation through advanced usage
• Covers .mgw/ directory structure, config.json options, all 15 commands with usage examples, GSD route explanation, dependency ordering (Kahn's algorithm), worktree isolation, status comment formats, cross-references, failure recovery, and FAQ
• Target audience: developers who have installed MGW and run /mgw:help but want deeper understanding

Closes #26

Milestone Context

• Milestone: v1 — Documentation & Developer Experience
• Phase: 5 — Extended Documentation
• Issue: 2 of 4 in milestone

Changes

• docs/
  • USER-GUIDE.md — New file (1311 lines) covering installation, configuration, command reference, workflow walkthroughs, GSD routes, dependency ordering, worktree isolation, status comments, cross-references, advanced usage, failure recovery, GSD integration, troubleshooting, and FAQ

Test Plan

• Verify all command names match current codebase (help.md, README.md)
• Verify .mgw/ directory structure matches state.md workflow
• Verify pipeline stages match state schema (new → triaged → planning → executing → verifying → pr-created → done)
• Verify GSD route names match VALID_GSD_ROUTES in template-loader.cjs
• Verify cross-refs link types match cross-refs schema in state.md
• Verify CLI flags match Commander.js options in bin/mgw.cjs
• Review for broken internal links to ARCHITECTURE.md and CONTRIBUTING.md