[autonomy-atomicity] Elastic-specific workflows are excluded from nav/catalog drift validation

## Autonomy / Atomicity Findings

### 1. Elastic-specific workflow docs are outside the nav/catalog drift guard
**Category:** Manual routing | Config hotspot
**File(s):**
- `scripts/check-nav-catalog.py:15-16, 28, 138-145`
- `mkdocs.yml:75-80`
- `docs/workflows/gh-agent-workflows.md:101-127`
- `.github/workflows/ci.yml:30-36`

**Problem:**
The docs nav/catalog consistency check intentionally excludes `estc-*` workflow slugs (`ELASTIC_SPECIFIC_PREFIX = "estc-"` and filtered sets in `main`). At the same time, Elastic-specific workflows are manually registered in both the MkDocs nav and the workflow catalog tables. This creates a concurrent-change hotspot with no CI drift protection for that section: parallel PRs that add/rename Elastic-specific workflows can merge with partial updates (nav-only or catalog-only) and pass this guard.

**Suggested fix:**
1. Include `estc-*` slugs in `check-nav-catalog.py` reachability validation (or add a dedicated Elastic-specific pass with equivalent enforcement).
2. Add tests in `tests/test_check_nav_catalog.py` that cover `estc-*` entries to prevent future regressions.
3. Keep one source-of-truth behavior for all workflow families (core + Elastic-specific) so doc registration changes fail fast when incomplete.

## Suggested Actions
- [ ] Remove or gate the `estc-*` exclusion in `scripts/check-nav-catalog.py` and enforce consistency for Elastic-specific workflows.
- [ ] Add regression tests proving `estc-*` entries must be reachable from nav/catalog.
- [ ] Document the expected registration/validation path for new Elastic-specific workflows in `gh-agent-workflows/DEVELOPING.md`.

Duplicate check notes:
- Not listed in `/tmp/previous-findings.json` as an open finding.
- Related to closed `#560`, but this is a narrower remaining gap: Elastic-specific workflows are currently exempted from the active drift guard.

---
[What is this?](https://ela.st/github-ai-tools) | [From workflow: Trigger Autonomy Atomicity Analyzer](https://github.com/elastic/ai-github-actions/actions/runs/31197036123)

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

Start with scripts/check-nav-catalog.py, especially the estc-* exclusion and filtered sets, then inspect mkdocs.yml, docs/workflows/gh-agent-workflows.md, and .github/workflows/ci.yml. Run the existing tests in tests/test_check_nav_catalog.py and add coverage for Elastic-specific entries. Done means inconsistent estc-* nav or catalog registration is detected by the validation guard.

Written by the indexing model from the issue text.