[Feat]: CI doc-drift guard for demos/ index parity

[rosspeili]

Feature Description

Blocked on: #195 — demos/ must exist before this is implementable.

Problem

tests/test_registry_docs.py guards catalog, examples index, and agent-loops parity. When #195 adds narrative notebooks under demos/, the same class of drift will appear: notebooks on disk vs rows in demos/README.md.

Direction (open)

Extend the doc-drift pattern (or sibling test module) to:

• Every demos/**/*.ipynb (or agreed path) appears in demos/README.md
• Orphaned index rows flag missing files
• Grandfather list only if explicitly justified (prefer *_common-style conventions)

Out of scope

• Running notebooks in CI
• Validating notebook content quality

Related: #195, tests/test_registry_docs.py

Rationale

Demos will be a third discovery surface (catalog, examples, demos). Without a guard, notebooks and index will diverge the way examples did before #183.

Affected paths (optional)

tests/test_registry_docs.py (or tests/test_demos_docs.py), demos/README.md, docs/TESTING.md, docs/demos.md (when #195 adds it)

Implementation Idea

Mirror existing examples parity tests; link from docs/TESTING.md as a fifth doc layer or subsection under demos.

Acceptance flexible — may stay open until first notebook merges in #195.