### Description
Migrate Gradle dependencies to version catalogs. This is the modern
style of managing dependencies. Confirmed with diff on `./gradlew
:app:dependencies` that no versions changed.
### Checklist
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### Description
Tweak agent instructions, with special focus on Claude.
### Checklist
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- [x] I have read the contribution guidelines:
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- [x] I have performed a self-review of my code, going through my
changes line by line and carefully considering why this line change is
necessary
- [x] I have run the automated code checks using `./gradlew checkstyle
lint spotbugsPlayDebug spotbugsDebug`
- [x] My code follows the style guidelines of the AntennaPod project:
https://antennapod.org/contribute/develop/app/code-style
- [x] I have mentioned the corresponding issue and the relevant keyword
(e.g., "Closes: #xy") in the description (see
https://docs.github.com/en/issues/tracking-your-work-with-issues/linking-a-pull-request-to-an-issue)
- [x] If it is a core feature, I have added automated tests
