[Docs] Add real CLI usage examples to README

[TheBoatyMcBoatFace]

Overview

The README’s “Usage” section shows uv run allusgov but no real-world examples. We should add 3–4 practical commands, e.g.:

# Scrape all sources and export JSON:
uv run allusgov all-steps --sources samgov,cisagov --export json

# Merge just SAM.gov and USA.gov:
uv run allusgov merge --merge-base samgov --sources samgov,usagov

Background

Concrete examples help users understand flags and workflows. They also serve as living documentation for common tasks.

References

• Example CONTRIBUTING.md (GitHub)
• GitHub Docs: Setting Up a Project for Contribution

Acceptance Criteria

• Under “Usage” in README.md, add at least 4 example commands covering:
  • Full pipeline (all-steps)
  • Spider only
  • Merge only
  • Export only
• Use fenced code blocks for clarity.
• Ensure examples run without errors (update any flags if outdated).