Drop obsolete scheme input from workflows

[leogdion]

Summary

Remove the ConfigKeyKit git-subrepo that is vendored into the MistKit repo at Packages/ConfigKeyKit (upstream brightdigit/ConfigKeyKit, .gitrepo branch main). Consume ConfigKeyKit as a normal remote SwiftPM dependency instead of a vendored local copy.

Background

MistKit currently carries a full vendored copy of this package under Packages/ConfigKeyKit (kept in sync via git subrepo). The example backends depend on it by local path rather than by URL:

• Examples/BushelCloud/Package.swift: .package(name: "ConfigKeyKit", path: "../../Packages/ConfigKeyKit")
• Examples/CelestraCloud/Package.swift: same local-path dependency

CI already pulls ConfigKeyKit remotely via the brightdigit/ConfigKeyKit/.github/actions/setup-configkeykit@main action, so the vendored copy is effectively only serving local-dev builds — a redundant second source of truth that has to be kept in sync by hand.

Proposed work (MistKit side)

• Delete the Packages/ConfigKeyKit subrepo directory and its .gitrepo.
• Repoint the example Package.swift dependencies from the local path to the remote package, e.g. .package(url: "https://github.com/brightdigit/ConfigKeyKit.git", ...) pinned to an appropriate tag/branch.
• Verify BushelCloud and CelestraCloud still build/test locally and in CI without the vendored copy.
• Drop any update-subrepo.sh/subrepo tooling references specific to ConfigKeyKit.

Notes

• The bulk of this work lands in the MistKit repo; this issue tracks it from the ConfigKeyKit side as requested. (Consider cross-linking or moving to brightdigit/MistKit if preferred.)
• Heads-up: there is an open MistKit PR (Drop obsolete scheme input from example workflows (#403) MistKit#413) that currently edits Packages/ConfigKeyKit/.github/workflows/ConfigKeyKit.yml. If the subrepo is being removed, that file should be dropped from PR #413 rather than modified.