notion-cli: reduce Node-backed sqlite runtime closure and expose reusable Node source CLI helper

Follow-up from #719 / #698 runtime investigation.

The supported notion sqlite ... path correctly runs under Node 24 because @overeng/notion-datasource-sync imports node:sqlite, which Bun 1.3.11 cannot resolve. Current Nix packaging handles this by wrapping notion sqlite to exec Node against the installed runtime workspace.

Remaining work:

• Add a reusable Node/source-runtime helper around the mk-pnpm-cli runtime workspace pattern instead of keeping ad hoc wrappers in package build files.
• Make installRuntimeWorkspace pruneable or add a production-runtime mode; current closure is large because it copies the restored workspace including dev/test surfaces.
• Consider exporting a narrow helper such as mkNodeSourceCliRuntime or mkPnpmCli from the flake for downstream md/sqlite-style CLIs.
• If notion-datasource-sync becomes standalone, give it its own Nix build/FOD/runtime dependency contract rather than only exposing it through notion-cli.passthru.datasourceSyncRuntime.

Evidence from local investigation: nix path-info -Sh reported the notion-cli/notion-datasource-sync runtime closure around 735 MiB, with the installed runtime workspace around 449 MiB.

Start by reading the flake's mk-pnpm-cli runtime workspace pattern, installRuntimeWorkspace, and the notion-cli.passthru.datasourceSyncRuntime packaging. Compare the current 735 MiB closure with the installed workspace, then define a reusable Node/source-runtime helper and a production-prunable runtime mode. Done means downstream CLIs can use the helper without ad hoc wrappers and the runtime closure is reduced or the dependency contract is standalone.