refactor(who): make --limit optional (unlimited default) and fix clippy sort lints

Change the `who` command's --limit flag from default=20 to optional,
so omitting it returns all results. This matches the behavior users
expect when they want a complete expert/workload/active/overlap listing
without an arbitrary cap.

Also applies clippy-recommended sort improvements:
- who/reviews: sort_by(|a,b| b.count.cmp(&a.count)) -> sort_by_key with Reverse
- drift: same pattern for frequency sorting

Adds Theme::color_icon() helper to DRY the stage-icon coloring pattern
used in sync output (was inline closure, now shared method).

src/cli/commands/who/reviews.rs

@@ -105,7 +105,7 @@ pub(super) fn query_reviews(
         })
         .collect();
 
-    categories.sort_by(|a, b| b.count.cmp(&a.count));
+    categories.sort_by_key(|b| std::cmp::Reverse(b.count));
 
     Ok(ReviewsResult {
         username: username.to_string(),