Migrate the WinGet package identity to grate-devs.grate

## Context

Follow-up to #796. The current WinGet package identifier is `erikbra.grate`. Changing it to `grate-devs.grate` creates a new WinGet package identity rather than renaming the existing package in place.

This is especially sensitive for installed portable packages and for MSI correlation/upgrade behavior.

## Scope

- Decide and document the compatibility strategy for existing `erikbra.grate` installations.
- Test migration behavior for both portable ZIP and MSI installations on Windows.
- Stabilize MSI upgrade identity and Add/Remove Programs metadata as needed; the current installer metadata still contains `erikbra` ownership and its upgrade identity needs explicit review.
- Generate and validate a new-package manifest for `grate-devs.grate`.
- Submit the new identity to `microsoft/winget-pkgs` and wait for it to be accepted.
- Coordinate retirement/removal of `erikbra.grate` with WinGet maintainers if appropriate.
- Change release automation from updating `erikbra.grate` to updating `grate-devs.grate` only after the new package exists.
- Preserve stable-only WinGet publication.

## Acceptance criteria

- [ ] The upgrade behavior of an installed `erikbra.grate` portable package is known and documented.
- [ ] The upgrade behavior of an installed `erikbra.grate` MSI package is known and documented.
- [ ] Any required MSI identity changes preserve a tested upgrade path.
- [ ] `grate-devs.grate` is accepted into the WinGet community repository.
- [ ] The release workflow submits later stable releases to `grate-devs.grate`.
- [ ] Users who cannot upgrade automatically have explicit uninstall/reinstall instructions.
- [ ] Documentation changes are coordinated with the distribution documentation sweep.

## References

- #796
- WinGet manifest identity: https://learn.microsoft.com/windows/package-manager/package/manifest
- Example identifier migration discussion: https://github.com/microsoft/winget-pkgs/issues/126959