ci(os-132): remove stale remote buildx mode

[jtoelke2]

Summary

Remove the stale remote BuildKit mode from the shared setup-buildx action now that all tracked callers use local Buildx on supported runners.

Related Issue

OS-132 / OS-49 Phase 8

Changes

• Removes the unused remote BuildKit action inputs and step from .github/actions/setup-buildx/action.yml.
• Drops redundant driver: local inputs from all current setup-buildx callers.
• Leaves buildkitd-config support in place for local builder registry mirror config.

Testing

• git diff --check
• YAML syntax parse for changed workflow/action files
• mise run pre-commit passes

mise run pre-commit was attempted. It failed on unrelated local workspace issues: markdownlint errors in untracked .codex-learning/*.md, plus existing local Rust test failure ssh::tests::launch_editor_returns_friendly_error_when_binary_missing. Rust check, clippy, Python tests, Helm lint, license check, formatting checks, and Mermaid lint completed successfully before the markdown task failure was reported.

Checklist

• Follows Conventional Commits
• Commits are signed off (DCO)

[github-actions]

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