Add doctor package with:
- Check interface for implementing health checks
- CheckContext for passing context to checks
- CheckResult and CheckStatus types
- Report with summary and pretty printing
- Doctor runner with Run() and Fix() methods
- BaseCheck and FixableCheck for easy check implementation
- CLI command: gt doctor [--fix] [--verbose] [--rig <name>]
Built-in checks will be added in:
- gt-f9x.5: Town-level checks (config, state, mail, rigs)
- gt-f9x.6: Rig-level checks (refinery, clones, gitignore)
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Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>
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