[autonomy-atomicity] Fixed quick-setup branch name creates cross-run branch contention risk

## Autonomy / Atomicity Findings

### 1. Fixed default branch in `quick-setup.sh` couples independent runs
**Category:** Config hotspot
**File(s):** `scripts/quick-setup.sh`, `README.md`

**Problem:**
`quick-setup.sh` hard-codes a shared default branch (`ai-gh-aw-setup`) for all runs. Concurrent developers/agents running setup against the same repo can collide on the same branch (`push` non-fast-forward) or accidentally accumulate unrelated commits/PR updates when the local branch already exists.

This creates an autonomy/atomicity blocker: independent setup changes are not isolated by default and must coordinate on shared branch state.

**Evidence:**
- `scripts/quick-setup.sh:11` sets `branch="ai-gh-aw-setup"`.
- `scripts/quick-setup.sh:24` documents the same default in CLI help.
- `scripts/quick-setup.sh:178-182` reuses existing local branch if present.
- `scripts/quick-setup.sh:269-270` always pushes and opens PR from that branch.
- `README.md:37-39` documents creation of the same fixed branch.

**Suggested fix:**
- Generate a unique default branch name (for example `ai-gh-aw-setup-(timestamp)-(rand)`).
- Before push, verify remote branch absence when default branch generation is used.
- Keep `--branch` for explicit deterministic naming.
- Optionally add `--reuse-branch` as an explicit opt-in for shared-branch behavior.

## Suggested Actions
- [ ] Make quick-setup default branch unique per run.
- [ ] Add a guard preventing accidental reuse of an existing remote branch by default.
- [ ] Preserve explicit `--branch` override for intentional fixed naming.
- [ ] Update README/help text to describe unique-by-default behavior and shared-branch opt-in.

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[What is this?](https://ela.st/github-ai-tools) | [From workflow: Trigger Autonomy Atomicity Analyzer](https://github.com/elastic/ai-github-actions/actions/runs/23552272715)

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