fix(ci): use portable fallbacks for bash-4 builtins and GNU sed in hook scripts

[jiengup]

Closes #3693

Rationale

Several hook scripts use mapfile (bash >= 4) and sed -i without backup extension (GNU extension). On macOS stock bash 3.2 and BSD sed, these fail, blocking contributors from committing.

What changed

• mapfile calls now guard with command -v mapfile and fall back to a while IFS= read -r loop.
• sed -i calls is replaced by the sed -i.bak && rm pattern already established in trailing-whitespace.sh.

Local Execution

• prek run -a

[github-actions]

Thanks for the PR. It is labeled S-waiting-on-review and queued for review.

Slash commands (own line, regular comment) move it around the queue:

• /ready - back to S-waiting-on-review after addressing feedback
• /author - flip to S-waiting-on-author while you finish changes
• /request-review @user-or-team - request a reviewer

See CONTRIBUTING.md for details.