[product-manager-impersonator] Quick setup shared defaults file

## Feature Idea

**Summary:** Add an optional `--defaults-file` mode to `scripts/quick-setup.sh` that applies one shared `with:` block (for example `additional-instructions` and `setup-commands`) to every installed trigger workflow.

## Why a Customer Would Want This
Teams that install multiple workflows currently have to edit each generated trigger file by hand to add the same customization values, which is repetitive and error-prone.

## Rough Implementation Sketch
- Add `--defaults-file ` to `scripts/quick-setup.sh` and validate the file exists.
- After each trigger download, inject a `with:` block into the `run` job before `secrets:` (same insertion point used by dogfood overrides).
- Support `--dry-run` by printing which defaults would be applied to which trigger files.
- Document the new option in `README.md` and `gh-agent-workflows/README.md` with a minimal example defaults file.

## Why It Won't Be That Hard
The setup script already centralizes trigger installation in one loop, and the repository already has proven YAML injection logic in `scripts/dogfood.sh` for inserting `with:` values. This is a small extension to existing patterns, not a new subsystem.

## Evidence
- `scripts/quick-setup.sh` (lines 23-30) exposes setup options, but none provide shared trigger-level `with:` customization.
- `scripts/quick-setup.sh` (lines 194-204) downloads each `trigger-*.yml` unchanged, so downstream customization is manual per file.
- `gh-agent-workflows/DEVELOPING.md` (lines 166-180) shows all workflows share `additional-instructions` and `setup-commands` inputs.
- `gh-agent-workflows/DEVELOPING.md` (line 208) explicitly expects consumers to copy and customize trigger examples.
- `scripts/dogfood.sh` (lines 75-91) already implements override insertion into trigger YAML, demonstrating a reusable approach.

## Duplicate Check
I checked existing open feature/enhancement ideas (including prior `[product-manager-impersonator]` issues) and did not find an open request for a quick-setup shared defaults file/injection flow.

---
[What is this?](https://ela.st/github-ai-tools) | [From workflow: Trigger Product Manager Impersonator](https://github.com/elastic/ai-github-actions/actions/runs/24505256568)

Give us feedback! React with 🚀 if perfect, 👍 if helpful, 👎 if not.