feat: add advisory doc drift preview

[marcusglee11]

Objective

Adds the first #122 advisory documentation drift sweep slice as a local dry-run issue preview.

No live issue creation, no cron activation, no doc mutation.

What changed

• Added doc_steward/drift_sweep_issue_creator.py
  • reuses existing entrypoint freshness detection
  • groups findings by drift class
  • computes stable per-class fingerprints
  • builds JSON + Markdown issue previews
  • writes timestamped and latest preview files
  • records a dry-run/non-mutating receipt
• Added doc_steward.cli drift-sweep-preview
  • requires explicit repo_root
  • supports --output-dir, --json, and --parent-link
• Added tests_doc/test_drift_sweep_issue_creator.py
  • 31 tests covering all six drift classes, clean state, fingerprint stability, JSON/Markdown shape, output dir behavior, parent link, safe repeated runs, and CLI integration

Verification

python3 -m pytest tests_doc/test_drift_sweep_issue_creator.py -q
31 passed

python3 -m pytest tests_doc -q
256 passed, 1 skipped

python3 scripts/workflow/quality_gate.py check --scope changed ... --json
passed=true for blocking checks

python3 -m doc_steward.cli drift-sweep-preview . --output-dir /tmp/doc_drift_preview --json
wrote JSON/Markdown preview files; current clean checkout has 0 findings

ea-dispatch review --base-ref origin/main
APPROVE

Local caveat: mypy is advisory and unavailable in this WSL environment; hosted CI will provide final readback.

Boundaries held

• No cron/scheduler activation.
• No live GitHub issue creation/comment/close from product code.
• No provider/model calls from product code.
• No credential access.
• No runtime/gateway changes.
• No doc auto-fix.
• No authority promotion/demotion.

Refs: #122, #141