fix(scripts): portable uppercase for branch-name acronym retention (bash 3.2) (#3192)

* fix(scripts): portable uppercase for branch-name acronym retention

Branch-name generation keeps short uppercase acronyms (e.g. "AI") by re-checking
the lowercased word against the original description with ${word^^}. That
parameter expansion is bash 4+ only; on macOS's default bash 3.2 it errors with
"bad substitution", so the acronym/short-word retention branch never matches and
those words are dropped ("go AI now" yields 001-now instead of 001-ai-now). Use
tr '[:lower:]' '[:upper:]' instead, which is portable.

Applies to both the core create-new-feature.sh and the git extension's
create-new-feature-branch.sh. The existing
test_branch_name_short_word_case_sensitivity / test_short_word_retention tests
cover this and now pass on bash 3.2 (CI runs on bash 4+/Linux, so they passed
there already).

(Disclosure: an AI coding agent surfaced the failure while running the suite on
macOS and pinned the root cause; fix written and reviewed by me.)

* fix(scripts): portability follow-ups from code review

- core create-new-feature.sh: match the acronym with `grep -qw` (POSIX
  whole-word) instead of `\b...\b` (GNU/BSD-only), matching the git extension
  and dropping a non-POSIX construct.
- lint: add a CI guard rejecting bash 4+ case-modification expansions in *.sh.
  shellcheck assumes bash 4+ from the shebang and can't flag them, and CI has no
  bash-3.2 lane, so this prevents silently re-shipping the macOS regression this
  PR fixes.
- update a stale PowerShell extension comment that cited the removed bash idiom.

(Disclosure: prompted by an AI code review of the PR; written and reviewed by me.)

scripts/bash/create-new-feature.sh

@@ -140,7 +140,7 @@ generate_branch_name() {
     local stop_words="^(i|a|an|the|to|for|of|in|on|at|by|with|from|is|are|was|were|be|been|being|have|has|had|do|does|did|will|would|should|could|can|may|might|must|shall|this|that|these|those|my|your|our|their|want|need|add|get|set)$"
     
     # Convert to lowercase and split into words
-    local clean_name=$(echo "$description" | tr '[:upper:]' '[:lower:]' | sed 's/[^a-z0-9]/ /g')
+    local clean_name=$(printf '%s' "$description" | tr '[:upper:]' '[:lower:]' | sed 's/[^a-z0-9]/ /g')
     
     # Filter words: remove stop words and words shorter than 3 chars (unless they're uppercase acronyms in original)
     local meaningful_words=()
@@ -152,8 +152,10 @@ generate_branch_name() {
         if ! echo "$word" | grep -qiE "$stop_words"; then
             if [ ${#word} -ge 3 ]; then
                 meaningful_words+=("$word")
+            # Keep short words that appear as an uppercase acronym in the original.
+            # Uppercase via tr and match with grep -w (both portable) rather than
+            # bash's 4+ "^^" case expansion (breaks on macOS bash 3.2) and \b (non-POSIX).
             elif printf '%s' "$description" | grep -qw -- "$(printf '%s' "$word" | tr '[:lower:]' '[:upper:]')"; then
-                # Keep short words if they appear as uppercase in original (likely acronyms)
                 meaningful_words+=("$word")
             fi
         fi