memoryd: the day and the month of a time with get, never a slice

Implemented-By: OpenCode session (model recorded in docs/implementer-log.md)

crates/memoryd/src/journal.rs

@@ -84,11 +84,7 @@ pub(crate) fn append(store: &Store, mut record: Record, now: Timestamp) -> Resul
     //    `serde_json::to_string(&record)`, then `store.append(&format!("edits/{month}.jsonl"),
     //    &format!("{line}\n"))?` where month is the first 7 characters of day. Ok(record.id).
     let rfc = now.to_rfc3339();
-    let day = if rfc.len() >= 10 {
-        &rfc[..10]
-    } else {
-        "0000-00-00"
-    };
+    let day = rfc.get(..10).unwrap_or("0000-00-00");
     let taken = records(store)?
         .iter()
         .filter(|r| r.id.starts_with(&format!("{day}-")))
@@ -97,7 +93,7 @@ pub(crate) fn append(store: &Store, mut record: Record, now: Timestamp) -> Resul
     let edits_dir = store.path("edits");
     std::fs::create_dir_all(&edits_dir).map_err(|e| format!("{}: {e}", edits_dir.display()))?;
     let line = serde_json::to_string(&record).map_err(|e| e.to_string())?;
-    let month = &day[..7];
+    let month = day.get(..7).unwrap_or("0000-00");
     store.append(&format!("edits/{month}.jsonl"), &format!("{line}\n"))?;
     Ok(record.id)
 }