agent: Rename @rules to @rule (#29881)

This is purely a cosmetic change, renamed `@rules` to `@rule` which
unifies the @mention experience (for files, threads etc. we also use
`@file`, `@thread` not `@files`, `@thread`). Would also make sense to
rename the rules picker to rule picker, but i do not wanna introduce
conflicts just for the purpose of re-naming.

Release Notes:

- N/A

crates/agent/src/active_thread.rs

@@ -722,7 +722,7 @@ fn open_markdown_link(
             }
         }),
         Some(MentionLink::Fetch(url)) => cx.open_url(&url),
-        Some(MentionLink::Rules(prompt_id)) => window.dispatch_action(
+        Some(MentionLink::Rule(prompt_id)) => window.dispatch_action(
             Box::new(OpenRulesLibrary {
                 prompt_to_select: Some(prompt_id.0),
             }),