feat(release): publish standalone openshell-gateway binaries

[drew]

Summary

Publish standalone openshell-gateway release artifacts from the existing openshell-server crate without changing the current container/runtime naming or openshell gateway start behavior.

Related Issue

Linear: https://linear.app/nvidia/issue/OS-85/release-standalone-openshell-gateway-binary-from-openshell-server

Changes

• add a shared openshell-server CLI entrypoint and a new openshell-gateway binary
• embed sqlite and postgres migrations so the standalone binary no longer depends on checkout-relative migration paths
• extend release-dev.yml and release-tag.yml to publish GNU Linux and macOS ARM64 openshell-gateway tarballs plus a dedicated checksum file
• update the support matrix and architecture docs for the new standalone gateway artifact

Testing

• mise run pre-commit passes
• Unit tests added/updated
• E2E tests added/updated (if applicable)
• cargo check -p openshell-server
• cargo test -p openshell-server cli::tests -- --nocapture
• cargo test -p openshell-server sqlite_connect_runs_embedded_migrations -- --nocapture
• release workflows and the macOS cross-build path were not run locally

Checklist

• Follows Conventional Commits
• Commits are signed off (DCO)
• Architecture docs updated (if applicable)

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