#3534: Update ad-hoc signing instructions in dev setup

Clarify instructions for using ad-hoc signing in the packaging script. Without this flag, macos will report building failure. <!-- greptile_comment --> <h2>Greptile Overview</h2> <h3>Greptile Summary</h3> This PR updates the macOS developer setup guide to clarify how to build/package the app without an Apple Developer ID certificate by explicitly opting into ad-hoc signing via `ALLOW_ADHOC_SIGNING=1` when running `scripts/package-mac-app.sh`. This aligns the dev-setup docs with the packaging/signing scripts’ behavior (ad-hoc signing is an explicit opt-in) and points developers to the macOS app README for deeper signing/run-mode troubleshooting. <h3>Confidence Score: 5/5</h3> - This PR is safe to merge; it’s a small, localized documentation clarification. - Only one markdown file is updated, and the new instruction (`ALLOW_ADHOC_SIGNING=1 ./scripts/package-mac-app.sh`) matches existing documented/scripted behavior elsewhere in the repo (e.g., mac signing docs and macOS app README). No code or config changes are introduced. - No files require special attention <!-- greptile_other_comments_section --> _{(2/5) Greptile learns from your feedback when you react with thumbs up/down!} **Context used:** - Context from `dashboard` - CLAUDE.md ([source](https://app.greptile.com/review/custom-context?memory=fd949e91-5c3a-4ab5-90a1-cbe184fd6ce8)) - Context from `dashboard` - AGENTS.md ([source](https://app.greptile.com/review/custom-context?memory=0d0c8278-ef8e-4d6c-ab21-f5527e322f13)) <!-- /greptile_comment -->