[product-manager-impersonator] Transactional rollback for quick-setup.sh failures

## Feature Idea

**Summary:** Add failure-safe rollback to `scripts/quick-setup.sh` so setup errors don’t leave users on a new branch with partially written workflow files.

## Why a Customer Would Want This
Teams use quick setup as the fastest onboarding path, but a mid-run failure currently leaves manual cleanup work (branch state + partial files). A transactional rollback would make first-run adoption safer and less frustrating.

## Rough Implementation Sketch
- Track initial branch and files created during execution (the script already collects `created_files`).
- Add a `trap`-based failure handler that, on error, removes newly created files and checks out the original branch.
- Add an opt-out flag (for example `--keep-partial`) for users who want to inspect partial state.
- Document rollback behavior in quick-setup docs/help text.

## Why It Won't Be That Hard
This is a localized Bash change in one script plus small docs updates. The script already has the key state needed (`created_files` and deterministic branch switch point), so rollback is mostly wiring cleanup into existing flow.

## Evidence
- `scripts/quick-setup.sh` exits immediately on command failure (`set -euo pipefail`) without cleanup today (`scripts/quick-setup.sh:9`).
- It switches/creates the setup branch before downloading workflow files (`scripts/quick-setup.sh:173-183`).
- It writes multiple workflow files incrementally (`scripts/quick-setup.sh:194-213`).
- It already tracks created files in-memory (`created_files` at `scripts/quick-setup.sh:186`, appends at `:202` and `:212`).
- Quick setup is positioned as the recommended install path in docs (`README.md:28-39`, `gh-agent-workflows/README.md:44-55`).

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