refactor: drop the AlchementaryLink keychain fallback and Go's Windows target format

Keychain reads no longer fall back to the AlchementaryLink service and
copy entries forward, and Windows Credential Manager entries use
keyring's own target naming instead of go-keyring's 'service:user'.
Credentials saved by Alchementary Link, or by earlier builds on Windows,
are no longer found and need re-entering.

rust/core/src/sessions.rs

@@ -1452,7 +1452,7 @@ mod tests {
         let _ = std::fs::remove_dir_all(&dir);
         std::fs::create_dir_all(&dir).unwrap();
         let store = Arc::new(Store::open(&dir.join("nexus.db")).unwrap());
-        let secrets = Arc::new(Secrets { vault: Vault::new(dir.join("vault")), keychain: Keychain::new("NexusTerminalTest", None), keychain_ok: false });
+        let secrets = Arc::new(Secrets { vault: Vault::new(dir.join("vault")), keychain: Keychain::new("NexusTerminalTest"), keychain_ok: false });
         (Manager::new(store, secrets, dir.join("recordings")), dir)
     }