Document check, describe, and print CLI subcommands

[ogolberg]

Summary

• Expand the README's CLI section to cover the check, describe, and print subcommands introduced in Add print and describe modes to the CLI #250 and Convert CLI modes into Clikt subcommands #254
• Show example invocations for each subcommand, including the gzipped TypeDescriptors proto format shared between describe and --platform-descriptors

Test plan

• Rendered README locally to verify formatting

🤖 Generated with Claude Code

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 90.94%. Comparing base (dcbde96) to head (dbc917e).

Additional details and impacted files

@@            Coverage Diff            @@
##               main     #258   +/-   ##
=========================================
  Coverage     90.94%   90.94%           
  Complexity      340      340           
=========================================
  Files            44       44           
  Lines          1137     1137           
  Branches        157      157           
=========================================
  Hits           1034     1034           
  Misses           52       52           
  Partials         51       51           

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.