[product-manager-impersonator] Bidirectional nav/catalog drift guard

## Feature Idea

**Summary:** Extend the existing docs nav/catalog consistency check to also fail when workflows appear in nav/docs but are missing from the catalog.

## Why a Customer Would Want This
Maintainers and downstream users rely on docs navigation and the workflow catalog to discover supported workflows. Today the check is one-way, so reverse drift can slip in and create confusing docs where workflows are visible in one place but absent in another.

## Rough Implementation Sketch
- Update `scripts/check-nav-catalog.py` to compute and report reverse drift (`reachable - catalog_slugs`) in addition to the current forward drift check.
- Keep Elastic-specific exclusions consistent with the current `estc-` handling.
- Add focused tests in `tests/test_check_nav_catalog.py` for reverse-drift failure and message output.
- Keep CI wiring unchanged since `.github/workflows/ci.yml` already runs this script.

## Why It Won't Be That Hard
The script already parses catalog slugs, nav slugs, and transitive doc mentions into sets, then performs a set-difference check. This is a small extension of existing logic plus a couple of tests in an established test file.

## Evidence
- Current check intent is one-way (catalog → nav reachability): `scripts/check-nav-catalog.py` lines 2-13.
- Current implementation only computes `catalog_slugs - reachable`: `scripts/check-nav-catalog.py` lines 147-157.
- Success message confirms only that one direction: `scripts/check-nav-catalog.py` line 159.
- CI already enforces this script, so feature lands in existing guardrail path: `.github/workflows/ci.yml` lines 30-36.
- Existing tests cover forward-missing behavior but not reverse drift: `tests/test_check_nav_catalog.py` lines 138-173.
- No close match found in `/tmp/previous-findings.json` (related-but-different items include #449 docs input table sync, #719 docs category conflict, #1695 quick-setup docs sync, #1809 rename parity).

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[What is this?](https://ela.st/github-ai-tools) | [From workflow: Trigger Product Manager Impersonator](https://github.com/elastic/ai-github-actions/actions/runs/31791813330)

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Start with scripts/check-nav-catalog.py, especially the existing catalog_slugs and reachable set-difference check, then read the related cases in tests/test_check_nav_catalog.py. Run that focused test file first. Done means reverse drift is detected with consistent estc- handling, the failure message reports it, and the existing CI invocation remains unchanged.

Written by the indexing model from the issue text.