rules_library: Default Rules tooltips imply per-thread attachment, but rules are re-applied per turn

[tredondo]

Reproduction steps

1. Open the Rules Library
2. Mouse over the Default Rules ℹ️ icon
3. Create a new rule
4. Mouse over the "Add to Default Rules" 📎 button

Current vs. Expected behavior

The tooltips for "Default Rules" in the rules library imply that rules are baked into a thread at creation time, but in reality they're re-injected into the system prompt on every turn (as revealed by testing and confirmed in the system prompt template).

Expected behavior

Both tooltips should answer 3 questions a user would actually ask:

1. Where does the rule go? → Into the system prompt of every thread (not just new ones).
2. When does an edit take effect? → On the next turn (no thread restart needed).
3. Given ^, How is the token count affected?

Zed version and system specs

Zed Dev 1.3.0 (debug)
Commit: bf423df