Rename examples/migration_v5/periodic_materialization/ to a product-aligned path

The deployment script, Terraform module, and customer playbook for periodic materialization live under `examples/migration_v5/periodic_materialization/`. The `migration_v5` segment is a development-history artifact; for product-release polish it should be renamed.

Suggested target: `examples/periodic_materialization/` or `examples/context_graph_deploy/`. The exact name should align with whatever the CLI ends up named (#245).

### Why this matters

After PR #243 lands, the only remaining `migration_v5` references in the codelab and blog are URL paths pointing at this folder. The directory name leaks development history into customer-facing docs.

### Scope

* Move `examples/migration_v5/periodic_materialization/` → `examples//`.
* Update path references in:
- `docs/blog/periodic_materialization.md`
- `docs/codelabs/periodic_materialization.md`
- `CHANGELOG.md` (forward-going entries)
- `examples/migration_v5_demo_notebook.ipynb` (verify whether it links to this folder)
- Any internal SDK imports or path constants
* Other artifacts under `examples/migration_v5/` (ontology.yaml, binding.yaml, reference_extractor.py, the demo notebook itself) are a separate question: they describe the migration-v5 demo specifically. Keep or rename together with #245 / #246.

### Out of scope here

The other `migration_v5_*` files at `examples/migration_v5/` root level (`mako_demo_agent.py`, `mako_core.ttl`, etc.) describe a different demo and can stay under their current path.