[information-architecture] PR Labeler is generated but missing from workflow discovery

## Information Architecture Findings

### 1. PR Labeler has installable docs but is absent from the workflow discovery paths
**Area:** Navigation / Grouping
**Component(s):** `gh-agent-workflows/pr-labeler/README.md`, `docs/hooks.py`, `mkdocs.yml`, `docs/workflows/gh-agent-workflows.md`

**Problem:** PR Labeler is an installable workflow, but users browsing the docs nav or the “Available workflows” catalog cannot discover it. The workflow README provides a Quick Install command and required configuration (`gh-agent-workflows/pr-labeler/README.md:1-15`, `gh-agent-workflows/pr-labeler/README.md:25-39`), and the docs hook generates a page for every workflow directory that has both `README.md` and `example.yml` (`docs/hooks.py:130-137`). However, the main Workflows nav lists the PR/event-driven workflows without PR Labeler (`mkdocs.yml:28-80`), and the catalog’s Human-coordinated/Event-driven tables also omit it while listing adjacent PR workflows such as PR Review, PR Review (Fork), Dependency Review, and Update PR Body (`docs/workflows/gh-agent-workflows.md:42-64`).

This leaves an existing PR automation feature effectively orphaned: a user looking for PR lifecycle automation can find review, fork review, CI detective, dependency review, and PR body updates, but not the PR labeling workflow.

**Suggested improvement:** Add PR Labeler to the Workflows nav and the “Available workflows” catalog near the other PR/event-driven workflows, with a short description and trigger summary.

## Suggested Actions
- [ ] Add `PR Labeler` to the `mkdocs.yml` Workflows navigation near the other PR workflows.
- [ ] Add `PR Labeler` to `docs/workflows/gh-agent-workflows.md` under the event-driven or PR workflow grouping.
- [ ] Ensure future generated workflow pages are checked for discovery coverage so installable workflows do not remain orphaned.

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[What is this?](https://ela.st/github-ai-tools) | [From workflow: Trigger Information Architecture](https://github.com/elastic/ai-github-actions/actions/runs/27974770320)

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Start with the workflow entries in mkdocs.yml and docs/workflows/gh-agent-workflows.md, then inspect the generation rules in docs/hooks.py and the PR Labeler README and example configuration. Add PR Labeler to the Workflows navigation and the appropriate event-driven catalog grouping with its description and trigger summary. Done means the installable workflow is discoverable in both documented paths without changing its implementation.

Written by the indexing model from the issue text.