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/store/mod.rs

@@ -48,7 +48,7 @@ use std::path::Path;
 use parse::classify_filename;
 
 /// Strip .md suffix from a key, handling both bare keys and section keys.
-/// "journal.md#j-2026" → "journal#j-2026", "identity.md" → "identity", "identity" → "identity"
+/// "identity.md" → "identity", "foo.md#section" → "foo#section", "identity" → "identity"
 pub fn strip_md_suffix(key: &str) -> String {
     if let Some((file, section)) = key.split_once('#') {
         let bare = file.strip_suffix(".md").unwrap_or(file);