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/subconscious/defs.rs

@@ -552,22 +552,16 @@ fn resolve(
             Some(Resolved { text, keys: vec![] })
         }
 
-        // latest_journal — the most recent journal entry for the journal agent
+        // latest_journal — the most recent EpisodicSession entry
         "latest_journal" => {
-            let text = store.nodes.get("journal")
-                .map(|n| {
-                    // Get the last entry (last ## section)
-                    let content = &n.content;
-                    content.rfind("\n## ")
-                        .map(|pos| content[pos..].to_string())
-                        .unwrap_or_else(|| {
-                            // Take the last 2000 chars if no ## found
-                            let start = content.len().saturating_sub(2000);
-                            content[start..].to_string()
-                        })
-                })
-                .unwrap_or_else(|| "(no previous journal entry)".to_string());
-            Some(Resolved { text, keys: vec!["journal".to_string()] })
+            let latest = store.nodes.values()
+                .filter(|n| n.node_type == crate::store::NodeType::EpisodicSession)
+                .max_by_key(|n| n.created_at);
+            let (text, keys) = match latest {
+                Some(n) => (n.content.clone(), vec![n.key.clone()]),
+                None => ("(no previous journal entry)".to_string(), vec![]),
+            };
+            Some(Resolved { text, keys })
         }
 
         _ => None,