`skip doctor` / `skip checkup` should have searchable error codes

When skip doctor / skip checkup fails, it's up to the user to try to fix it. Often, they respond by posting on Slack or on the Github discussion forum. In the best case, they include their log, and they have to wait for someone to respond to them. (In the worst case, they forget to include the log, and their report is unactionable.)

Instead, skip doctor and skip checkup should detect common installation issues and report them with a searchable error code, e.g.

[SKIP-DOCTOR-ERR-CODE-1] Gradle not found

Those error codes should show up on https://skip.dev/docs/help/ (which is currently mostly nearly blank).

The goal would be for the user (and/or an LLM) to be able to clearly identify not just what the problem is, but how exactly to fix it, without human interaction.

Begin at the skip doctor and skip checkup command entry points and review how common installation failures are currently reported. Read the help page at skip.dev/docs/help/ and define the error-code coverage and fix guidance needed for each documented failure. Done means the commands emit searchable codes and the help page explains how to resolve them.