Add Who This Is For and Troubleshooting sections to README

[snipcodeit]

Summary

• Added "Who This Is For" section after "Why I Built This" describing target audience (solo devs and small teams using Claude Code), a decision table for when to use MGW vs Claude Code directly, and prerequisites
• Added comprehensive "Troubleshooting" section before "Contributing" covering GitHub CLI auth, GSD installation, .mgw/ state drift and corruption, worktree cleanup, API rate limiting, common error table, and uninstall instructions
• Both sections integrate with the existing README voice and flow without modifying existing content

Closes #27

Milestone Context

• Milestone: v1 — Documentation & Developer Experience
• Phase: 6 — README Final Polish
• Issue: 3 of 4 in milestone

Changes

• README.md — Added "Who This Is For" section (31 lines) between "Why I Built This" and "Commands"; added "Troubleshooting" section (116 lines) between "Acknowledgments" and "Contributing"

Test Plan

• Verify "Who This Is For" section renders correctly on GitHub with table formatting
• Verify "Troubleshooting" section renders correctly with all code blocks and error table
• Confirm no existing content was modified (only additions)
• Check all links in new sections resolve correctly (Claude Code docs, GSD repo, GitHub CLI)
• Verify README table of contents / section ordering flows naturally