[information-architecture] PR Labeler quick install hides required label customization

## Information Architecture Findings

### 1. PR Labeler quick install is separated from the required label picker
**Area:** Action placement / Picker placement
**Component(s):** `gh-agent-workflows/pr-labeler/README.md`, `gh-agent-workflows/pr-labeler/example.yml`, `.github/workflows/gh-aw-pr-labeler.md`
**Problem:** The PR Labeler README puts the copy/paste Quick Install command first (`gh-agent-workflows/pr-labeler/README.md:9-17`), but the workflow requires callers to choose repository-specific labels. That required `classification-labels` configuration is documented only afterward (`gh-agent-workflows/pr-labeler/README.md:25-39`) and again in the input table (`gh-agent-workflows/pr-labeler/README.md:73-83`). The example copied by Quick Install hardcodes `small_boom,medium_boom,big_boom` (`gh-agent-workflows/pr-labeler/example.yml:16-33`), while the reusable workflow marks `classification-labels` as required (`.github/workflows/gh-aw-pr-labeler.md:36-39`).

Users following the primary setup action can install an uncustomized label taxonomy before seeing the required picker/configuration step, which is especially confusing because labels must already exist in the target repository (`gh-agent-workflows/pr-labeler/README.md:38-39`).

**Suggested improvement:** Move the label-selection requirement into the Quick Install section, or change Quick Install into a short ordered setup flow: copy the trigger, replace `classification-labels` and `additional-instructions` with repo-specific labels, then ensure those labels exist before enabling the workflow.

## Suggested Actions
- [ ] Update `gh-agent-workflows/pr-labeler/README.md` so Quick Install includes the required label customization step before users copy or enable the workflow.
- [ ] Replace or annotate the hardcoded `small_boom,medium_boom,big_boom` example so it is clearly a placeholder taxonomy, not a ready-to-use default.
- [ ] Add an explicit reminder near the install command that all configured labels must already exist in the target repository.

---
[What is this?](https://ela.st/github-ai-tools) | [From workflow: Trigger Information Architecture](https://github.com/elastic/ai-github-actions/actions/runs/28609654568)

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

Start with the Quick Install section in gh-agent-workflows/pr-labeler/README.md and compare it with example.yml and .github/workflows/gh-aw-pr-labeler.md. Update the setup flow so label customization and the existing-label requirement appear before enabling the workflow, and make the sample taxonomy clearly a placeholder. Done means the README, example, and reusable-workflow documentation present consistent setup guidance.

Written by the indexing model from the issue text.