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/drift.rs

@@ -382,7 +382,7 @@ fn extract_drift_topics(description: &str, notes: &[NoteRow], drift_idx: usize)
     }
 
     let mut sorted: Vec<(String, usize)> = freq.into_iter().collect();
-    sorted.sort_by(|a, b| b.1.cmp(&a.1));
+    sorted.sort_by_key(|b| std::cmp::Reverse(b.1));
 
     sorted
         .into_iter()