Document XDG trust config

[MikeMcQuaid]

Closes #22504
It was already working, just not clearly documented.

• Explain the trust.json path in brew trust and brew untrust help so users can find the stored policy file.
• Keep documenting livecheck_watchlist.txt consistently with HOMEBREW_USER_CONFIG_HOME startup behaviour.

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[github-code-quality]

Code Coverage Overview

Languages: Ruby

Ruby / code-coverage/simplecov

The overall coverage in the document-xdg-trust-c... branch is 75%. The coverage in the main branch is 78%.

Show a code coverage summary of the most impacted files.

File	main 63f2962	document-xdg-trust-c... 9c3868d	+/-
cask/info.rb	96%	0%	-96%
cask/installer.rb	91%	28%	-63%
cask/artifact/a...ct_uninstall.rb	89%	32%	-57%
cask/audit.rb	63%	25%	-38%
cask/cask_loader.rb	96%	65%	-31%
bundle/extensions/flatpak.rb	37%	94%	+57%
extend/os/linux/diagnostic.rb	0%	67%	+67%
os/linux/elf.rb	0%	86%	+86%
extend/os/linux/sandbox.rb	0%	95%	+95%
extend/os/linux...ystem_config.rb	0%	99%	+99%

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_{Updated June 01, 2026 18:57 UTC
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