Add Format rules should be non-destructive

[swiftal64]

Summary

Add "Format rules should be non-destructive" as a guiding tenet to prevent data loss and establish clear expectations for rule development.

Reasoning

We previously experienced data loss from a destructive formatting rule that had to be removed. This tribal knowledge isn't documented, creating risks as new contributors may unknowingly introduce destructive rules.

Codifying this principle will:

1. Increase the likelihood that destructive rules are blocked during the review process;
2. Prevent hashing out the debate again, as there's an easily linkable reference to the tenet; and
3. Protect user workflows across all trigger contexts (IDE shortcuts, pre-commit hooks, CI/CD)

As linting triggers broaden, the blast radius of destructive behavior increases, making this formalization critical for safe, repeatable formatting that users can trust.

[calda]

Thanks, will discuss this with the style team to see if we want to list this as a guiding tenet