Add bsk CLI auto update support

[BB-fat]

Summary

• Add a native bsk update command that checks GitHub release metadata, downloads the current-platform archive, verifies SHA-256, extracts the CLI binary, and replaces or stages the update.
• Extend the CLI release manifest to include per-asset checksums and wire the release workflow to render checksum-aware version.json.
• Add cached background update checks for normal CLI commands, skipping --json, --quiet, daemon, and update commands to avoid output pollution.

Test plan

• node --test scripts/render-version-json.test.mjs
• cargo fmt --check
• cargo test -p bsk --locked
• cargo clippy -p bsk --all-targets -- -D warnings