Convert CLI modes into Clikt subcommands

[ogolberg]

Summary

• Replace the --mode flag with check / print / describe subcommands following the Clikt subcommands guide
• Share --project-classes / --libraries across check and describe via a SourcesOptions : OptionGroup, per the advanced.md pattern for sharing options
• Use Clikt's option().required() for mandatory flags (--output, --platform-descriptors) instead of hand-rolled error() checks

Test plan

• ./gradlew :cli:test :cli:spotlessCheck passes
• Integration tests updated to invoke check / print / describe as subcommands, including the describe → print round-trip
• Error-path tests updated to assert on CliktError from .required() where applicable

🤖 Generated with Claude Code

[codecov]

Codecov Report

❌ Patch coverage is 90.62500% with 3 lines in your changes missing coverage. Please review.
✅ Project coverage is 90.94%. Comparing base (1afcd10) to head (f2988a6).

Files with missing lines	Patch %	Lines
.../com/toasttab/expediter/cli/ExpediterCliCommand.kt	90.62%	0 Missing and 3 partials ⚠️

Additional details and impacted files

@@             Coverage Diff              @@
##               main     #254      +/-   ##
============================================
- Coverage     90.95%   90.94%   -0.02%     
+ Complexity      365      340      -25     
============================================
  Files            44       44              
  Lines          1139     1137       -2     
  Branches        155      157       +2     
============================================
- Hits           1036     1034       -2     
  Misses           52       52              
  Partials         51       51              

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