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.)

.github/workflows/lint.yml

@@ -54,3 +54,16 @@ jobs:
       # (notably SC2155). Tighten in a follow-up after cleanup.
       - name: Run shellcheck on shell scripts
         run: git ls-files -z -- '*.sh' | xargs -0 shellcheck --severity=error
+
+      # macOS ships bash 3.2, where bash 4+ case-modification parameter
+      # expansions error with "bad substitution". shellcheck assumes bash 4+
+      # from the shebang and cannot flag these, so guard explicitly; use tr
+      # for portable case conversion.
+      - name: Reject bash 4+ case-modification expansions
+        run: |
+          matches=$(git ls-files -z -- '*.sh' | xargs -0 grep -nE '\$\{[A-Za-z_][A-Za-z0-9_]*(\[[^]]*\])?(\^\^?|,,?|~~?|@[UuLl])[^}]*\}' || true)
+          if [ -n "$matches" ]; then
+            echo "Found bash 4+ case-modification expansion(s); use tr for portability (macOS ships bash 3.2):"
+            echo "$matches"
+            exit 1
+          fi