feat(cli): add release update check - #16

Summary

• add an explicit sc update command without adding startup network traffic
• prefer authenticated GitHub CLI release lookup for the private repo, with a token-aware REST fallback
• add JSON output for scripts and point available updates to subc sc install
• document the command and changelog entry

Verification

• cargo fmt --all -- --check
• cargo clippy --locked --workspace --all-targets -- -D warnings
• cargo test --locked --workspace
• cargo +1.89.0 check --locked --workspace
• live sc update and sc update --json against release v0.1.2