docs: add USAGE.md — complete user guide with scenarios and troubleshooting

[sagar-grv]

User documentation — USAGE.md

Adds a comprehensive 558-line user guide covering every scenario, all generated files, deep-dive troubleshooting, and FAQ.

Sections

1. Quick Start — 3 steps with real terminal output
2. Scenario: New / empty project
3. Scenario: Existing Node.js app
4. Scenario: App with deploy URL (health.yml explained)
5. Scenario: Monorepo
6. Scenario: GitLab CI
7. Scenario: Bitbucket Pipelines
8. What each generated file does (all 8 files explained)
9. Commands reference table
10. Troubleshooting — 13 items (Node version, npx cache, permissions, Windows PATH, proxy, monorepo detection, CodeQL, and more)
11. FAQ — 7 questions

README.md change

Added one line after Quick Start linking to USAGE.md.