Add fs::MTime newtype to encourage != instead of > (#20830)

See ["mtime comparison considered
harmful"](https://apenwarr.ca/log/20181113) for details of why
comparators other than equality/inequality should not be used with
mtime.

Release Notes:

- N/A

crates/assistant/src/context_store.rs

@@ -770,7 +770,7 @@ impl ContextStore {
                         contexts.push(SavedContextMetadata {
                             title: title.to_string(),
                             path,
-                            mtime: metadata.mtime.into(),
+                            mtime: metadata.mtime.timestamp_for_user().into(),
                         });
                     }
                 }