fix hint text per argus review: group into subdirs, not split into more top-level files

argus caught that the previous wording ("splitting a long-lived file
into dated pieces") suggested a remedy that increases top-level entry
count unless the pieces land in a subdirectory - wrong advice for the
exact metric this watch counts.

Also added mara's ask: explicitly note the agent can leave the todo
open and act on it later, no pressure to resolve immediately.

hive-agent/src/state_entry_watch.rs

@@ -95,9 +95,10 @@ fn summary_for(count: u64) -> Option<String> {
     let bucket = count / BUCKET * BUCKET;
     Some(format!(
         "your state dir has {bucket}+ entries at the top level — worth a look if it's \
-         gotten unwieldy, no pressure. Splitting a long-lived file into dated pieces or \
-         moving one-off scratch out of the top level are both fine ways to trim it; \
-         nothing here is a hard limit and nothing gets deleted for you."
+         gotten unwieldy, no pressure. Grouping related top-level entries into subdirectories \
+         is what actually lowers this count; nothing here is a hard limit and nothing gets \
+         deleted for you. No need to act on this now — leave the todo open and come back to \
+         it whenever."
     ))
 }