Replaces the `@rule` autocomplete entry in the agent chat input with
`@skill`, since Zed Rules are being deprecated in favor of Skills.
Behavior matches the old `@rule` flow: selecting a skill inserts a Skill
mention crease that loads the skill file as context, and the entry only
appears when there are available skills.

The underlying `MentionUri::Rule` parsing/handling is left in place so
previously saved rule mentions still load correctly — only the
autocomplete surface area was migrated.

Release Notes:

- Replaced the `@rule` autocomplete entry with `@skill` in the agent
chat input.