Add docs-frontmatter-sweep agentic workflow (#123)

New scheduled / on-demand sweep that audits frontmatter quality across a
docs corpus on a rotating slice each run. Combines two existing skills
(`docs-frontmatter-audit` for required-field validation and
`docs-frontmatter-description` for SEO-quality descriptions) and emits a
single labeled fix-issue with structured YAML findings consumable by a
future fix-agent.

Establishes the sweep pattern shared by the rest of this family:

- Deterministic rotating slice via `hash(path) mod N == iso_week mod N`,
  with `N` auto-derived from corpus size.
- Pages modified in the last 7 days included regardless of rotation.
- Slice files copied to `/tmp/gh-aw/sweep-data/scope/` so skills can
  operate without affecting the repo.
- Fix-issues filed cross-repo to `elastic/docs-content-internal` (via
  `safe-outputs.create-issue.target-repo`) to keep AI triage findings
  out of public docs repos. Consumers must configure
  `DOCS_FIX_ISSUES_TOKEN` with `issues:write` on the internal repo.
- `close-older-issues: true` keeps the tracker clean across runs.

Part of phase 3 of elastic/docs-content-internal#949.

Co-authored-by: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: theletterf