Add fix command registration and update agent UI label handling (#1387)

Adds a fix command to the stack cli 


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## Summary by CodeRabbit

* **New Features**
* Added a CLI "fix" command to submit Stack Auth errors (flag, stdin, or
interactive), confirm before applying changes, show a customizable
progress label, and produce a final markdown report with Error, Files
changed, and Solution.
* Added a CLI "doctor" command to analyze projects (framework override,
output directory, JSON output), run framework-specific checks, validate
env and config, and exit non-zero on failures.
* **Tests**
  * Added comprehensive end-to-end tests for the doctor command.
<!-- end of auto-generated comment: release notes by coderabbit.ai -->

Author: aadesh18