[UX] `git-copilot review` — the core command — exits immediately with no output

[coderabbitai]

Summary

New and existing users who follow the README Quick Start guide and run git-copilot review receive only:

Starting code review... (Not yet implemented)

The tool's primary value proposition is completely inaccessible.

User Impact

• First-time users will immediately uninstall after seeing this
• The README Quick Start section (steps 1–3) is entirely non-functional
• No workaround exists — only git-copilot export --format ... works

Recommended Fix

Implement the review command as the top priority. It should:

1. Detect the active provider from config (with a clear error if unconfigured)
2. Run the ReviewWorkflow (infrastructure already exists)
3. Display results in the terminal using the Ink/Blessed UI

Backlink: #1