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/bin/memory-search.rs

@@ -150,19 +150,12 @@ fn load_or_create_cookie(dir: &Path, session_id: &str) -> String {
 }
 
 fn generate_cookie() -> String {
-    let out = Command::new("head")
-        .args(["-c", "12", "/dev/urandom"])
-        .output()
+    const ALPHA: &[u8] = b"0123456789abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ";
+    let mut buf = [0u8; 16];
+    fs::File::open("/dev/urandom")
+        .and_then(|mut f| io::Read::read_exact(&mut f, &mut buf))
         .expect("failed to read urandom");
-    out.stdout.iter()
-        .map(|b| {
-            let idx = (*b as usize) % 62;
-            if idx < 10 { (b'0' + idx as u8) as char }
-            else if idx < 36 { (b'a' + (idx - 10) as u8) as char }
-            else { (b'A' + (idx - 36) as u8) as char }
-        })
-        .take(16)
-        .collect()
+    buf.iter().map(|b| ALPHA[(*b as usize) % ALPHA.len()] as char).collect()
 }
 
 fn load_seen(dir: &Path, session_id: &str) -> HashSet<String> {