fix(npm): hide native CLI consoles on Windows

[Oscar-Williams]

What

Pass windowsHide: true to both npm launcher paths that start or synchronously invoke the native Mnemon binary. Add Node regression coverage for the default and explicit override behavior.

Why

Closes #143. GUI-hosted agent harnesses on Windows otherwise create a visible console window on every mnemon invocation, which is especially disruptive for periodic status polling.

Validation

• npm test --prefix npm/cli (13 passed on Node 24; the mock uses the Node 22-compatible namedExports option)
• node --check for the changed launcher and test modules
• git diff --check
• Current fork CI run 35971834668 is action_required pending maintainer approval.
• make test was not run locally because this Windows checkout has no Go toolchain; the existing CI workflow runs it on Ubuntu and Windows.

Checklist

• Deterministic tests pass (make test)
• Relevant E2E/process/Docker boundaries are unaffected (npm launcher-only change)
• New/changed behavior is covered by tests
• Documentation update is not applicable (no user-facing command or configuration change)
• No release-note impact; the npm launcher remains behaviorally identical except for suppressing Windows console creation