[WIP] Refactor repeated safe-output handler scaffolding across JS handlers

[Copilot]

• Analyze all 35 count-gated handlers and categorize by increment position
• Create actions/setup/js/handler_factory.cjs with createCountGatedHandler utility
• Create actions/setup/js/handler_factory.test.cjs with focused max-limit tests (7 tests)
• Migrate cohort 1: add_labels, remove_labels
• Migrate cohort 2: assign_to_user, unassign_from_user
• Migrate remaining 20 handlers: close_issue, close_pull_request, close_discussion, add_reviewer, dispatch_workflow, hide_comment, call_workflow, add_comment, create_pull_request, autofix_code_scanning_alert, assign_milestone, create_project, create_project_status_update, create_discussion, link_sub_issue, mark_pull_request_as_ready_for_review, resolve_pr_review_thread, set_issue_type, push_to_pull_request_branch, missing_data
• Run make fmt-cjs to format
• Verify all existing handler tests still pass (773+ tests across all migrated handlers)
• Fix 3 stale comments flagged by code review (non-blocking)

[pelikhan]

@copilot finish work and push changes