M6.3: Write comprehensive README

[bjcorder]

Labels: type:docs, priority:critical, status:blocked,
milestone:M6, phase:10

Summary

Write the project README with problem statement, installation,
quickstart, and limitations.

Acceptance criteria

• README.md includes:
  • One-paragraph problem statement
  • Conceptual model: commit ancestry, not version coordinates
  • Installation: download from releases, or go install
  • Quickstart: make demo
  • How to add a real fork (forks.yaml schema with example)
  • Limitations and what's not in the PoC
  • License: Apache 2.0
  • Link to design docs
• Includes badges: build status, Go version, license, latest release
• Includes an asciicast or animated GIF of the demo (optional but
  preferred)

Dependencies

Depends on: #29

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