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/context_picker/completion_provider.rs

@@ -455,7 +455,7 @@ impl ContextPickerCompletionProvider {
         editor: Entity<Editor>,
         context_store: Entity<ContextStore>,
     ) -> Completion {
-        let new_text = MentionLink::for_rules(&rules);
+        let new_text = MentionLink::for_rule(&rules);
         let new_text_len = new_text.len();
         Completion {
             replace_range: source_range.clone(),