refactor: extract Store methods, clean up shell-outs

- Add Store::upsert() — generic create-or-update, used by cmd_write
- Add Store::insert_node() — for pre-constructed nodes (journal entries)
- Add Store::delete_node() — soft-delete with version bump
- Simplify cmd_write (20 → 8 lines), cmd_node_delete (16 → 7 lines),
  cmd_journal_write (removes manual append/insert/save boilerplate)
- Replace generate_cookie shell-out to head/urandom with direct
  /dev/urandom read + const alphabet table

main.rs: 1137 → 1109 lines.

src/capnp_store.rs

@@ -491,6 +491,48 @@ impl Store {
         Ok(())
     }
 
+    /// Upsert a node: update if exists (and content changed), create if not.
+    /// Returns: "created", "updated", or "unchanged".
+    pub fn upsert(&mut self, key: &str, content: &str) -> Result<&'static str, String> {
+        if let Some(existing) = self.nodes.get(key) {
+            if existing.content == content {
+                return Ok("unchanged");
+            }
+            let mut node = existing.clone();
+            node.content = content.to_string();
+            node.version += 1;
+            self.append_nodes(std::slice::from_ref(&node))?;
+            self.nodes.insert(key.to_string(), node);
+            Ok("updated")
+        } else {
+            let node = Store::new_node(key, content);
+            self.append_nodes(std::slice::from_ref(&node))?;
+            self.uuid_to_key.insert(node.uuid, node.key.clone());
+            self.nodes.insert(key.to_string(), node);
+            Ok("created")
+        }
+    }
+
+    /// Soft-delete a node (appends deleted version, removes from cache).
+    pub fn delete_node(&mut self, key: &str) -> Result<(), String> {
+        let node = self.nodes.get(key)
+            .ok_or_else(|| format!("No node '{}'", key))?;
+        let mut deleted = node.clone();
+        deleted.deleted = true;
+        deleted.version += 1;
+        self.append_nodes(std::slice::from_ref(&deleted))?;
+        self.nodes.remove(key);
+        Ok(())
+    }
+
+    /// Insert a fully constructed node (for journal entries, imports, etc.)
+    pub fn insert_node(&mut self, node: Node) -> Result<(), String> {
+        self.append_nodes(std::slice::from_ref(&node))?;
+        self.uuid_to_key.insert(node.uuid, node.key.clone());
+        self.nodes.insert(node.key.clone(), node);
+        Ok(())
+    }
+
     /// Create a new node with defaults
     pub fn new_node(key: &str, content: &str) -> Node {
         Node {