journal: remove all stringly-typed key patterns, use NodeType

- journal_new: key is slugified title (agent names things properly)
- journal_tail: sort by created_at (immutable), not timestamp (mutable)
- journal_update: find latest by created_at
- {{latest_journal}}: query by NodeType::EpisodicSession, not "journal" key
- poc-memory journal write: requires a name argument
- Removed all journal#j-{timestamp}-{slug} patterns from:
  - prompts.rs (rename candidates)
  - graph.rs (date extraction, organize skip list)
  - cursor.rs (date extraction)
  - store/mod.rs (doc comment)
- graph.rs organize: filter by NodeType::Semantic instead of key prefix
- cursor.rs: use created_at for date extraction instead of key parsing

Co-Authored-By: Kent Overstreet <kent.overstreet@linux.dev>

src/hippocampus/cursor.rs

@@ -89,17 +89,13 @@ pub fn digest_parent(store: &Store, key: &str) -> Option<String> {
         if node.timestamp > 0 {
             dates.push(store::format_date(node.timestamp));
         }
-        // Extract date from key patterns like "journal#2026-03-03-..." or "journal#j-2026-03-13t..."
-        if let Some(rest) = key.strip_prefix("journal#j-").or_else(|| key.strip_prefix("journal#"))
-            && rest.len() >= 10 {
-                let candidate = &rest[..10];
-                if candidate.chars().nth(4) == Some('-') {
-                    let date = candidate.to_string();
-                    if !dates.contains(&date) {
-                        dates.push(date);
-                    }
-                }
+        // Extract date from created_at timestamp
+        if node.created_at > 0 {
+            let created_date = store::format_date(node.created_at);
+            if !dates.contains(&created_date) {
+                dates.push(created_date);
             }
+        }
         for date in &dates {
             for prefix in [&format!("daily-{}", date), &format!("digest#daily#{}", date)] {
                 for (k, n) in &store.nodes {