Add Windows smoke test for winget install

Parent: #121

End-to-end smoke test in CI on `windows-latest` exercising the install path operators will actually use.

## Definition of done

GH Actions job that, on a manual dispatch and on release-candidate tags:

1. Validates the local winget manifest (__W1__) against the candidate installer (__W2__).
2. Installs from the local manifest via `winget install --manifest`.
3. Asserts the `FalloutAgent` service exists and starts (__W3__).
4. Runs a mocked coordinator handshake — agent reaches a local stub, returns "enrolled".
5. Uninstalls via `winget uninstall Fallout.Agent`; asserts the service is gone and `%ProgramData%\Fallout\Agent\` is cleaned per spec.
6. Job exits 0 only if all six steps succeed.