Support for redacting private/sensitive information

# Description of the new feature/enhancement

I'd like the ability to automatically mask sensitive information in terminal windows. Masking application output would be great; masking user input (e.g., passwords on command line) would be nice as well.

This would be useful when screen sharing, demos, and when recording screencasts.

[Warp does this](https://mkennedy.codes/posts/hide-those-terminal-secrets/).

# Proposed technical implementation details (optional)

Terminal needs to know what to mask. At the very least it should have a user-editable series of regexes to detect potentially sensitive information. Even better would be integration with GitHub's existing secret scanning code, masking anything that matches GitHub's token patterns.

Masked information could use a specific Unicode character (or one of a few similar Unicode characters), so it's as obvious as possible that the output was masked.

Ideally, copying a masked value would copy the original plaintext, so copy/paste works the same as it always does, ignoring the masking.

The issue names no files, tests, or entry points. Start by clarifying whether masking applies to terminal output, user input, or both, then identify the affected terminal rendering, input, and clipboard paths and define how users configure masking and how completion will be tested.

Written by the indexing model from the issue text.