Basic encounter triggers

> [!NOTE]
> As a user, I want the system to automatically manage events triggered in an encounter. For example: the system should remove conditions when their duration expires, apply fast-healing for monsters, remind the user about the legendary actions, etc.

This issue should be the base for encounter triggers. Test this feature with the duration of the condition.

Start by tracing the encounter event handling and condition-duration behavior; the issue names no files or tests. Use the condition-duration case as the first validation target. Done means the encounter trigger foundation manages an expired condition automatically, with the behavior covered by a test.

Written by the indexing model from the issue text.