fix: patch update.sh to fully resolve Shellcheck SC2155 & SC1083 warnings

[maruf-pfc]

Overview

This PR resolves the final instances of Shellcheck verification failures that were failing our main GitHub Action CI when scanning the repository root.

What's Changed

When the CI scanned the ./update.sh script, it flagged two issues affecting bash stability and POSIX standard definitions:

• SC1083 (Literal Braces): The git rev-parse @{u} execution contained literal { } characters in the unquoted string @{u}. Though usually parsed correctly by bash, strictly enforcing explicit quotes ("@{u}") is the certified POSIX bash standard for stability.
• SC2155 (Masking Subshell Return Values): The script originally contained explicit assignment inside the local declaration (local LOCAL=$(...)). If the subshell ($(...)) fails, the local constructor immediately overrides the negative return code with its own 0 (success). Because of this, we split the declarations strictly:

  local LOCAL
  LOCAL=$(git rev-parse HEAD 2>/dev/null)