fix: limit markdown linter hook to staged files only

Limit markdown linter hook that is being run pre-commit, to staged files only

  - Change final linting check from '**/*.md' to staged files only
  - Prevents hook failure on unstaged markdown files in project
  - Maintains git staging area respect and workflow consistency
  - Improves developer experience by focusing on actual changes

The hook was incorrectly scanning all markdown files after auto-fixing
staged files, causing failures on unrelated unstaged files like agent
definitions or documentation drafts.

Author: RickCogley

adhoc/git/hooks/20-lint-markdown

@@ -39,8 +39,8 @@ if command -v markdownlint-cli2 >/dev/null 2>&1; then
             stage_changes "$md_files"
         fi
 
-        # Now run final linting check on all markdown files (for project consistency)
-        if markdownlint-cli2 "**/*.md"; then
+        # Now run final linting check on staged markdown files only
+        if echo "$md_files" | xargs markdownlint-cli2; then
             log_success "Markdown linting passed"
         else
             log_error "Markdown linting failed - manual fixes needed"