Add AccessGrant and CalendarAvailability ontology schemas

[AlexTourski]

Summary

Adds two proposed W3DS JSON Schema draft-07 ontologies:

• AccessGrant for canonical owner-to-grantee authorization and non-authoritative discovery references.
• CalendarAvailability for explicit owner-defined booking windows.

The schemas use newly generated UUIDv4 identifiers:

• AccessGrant: 15d24c04-a4f3-4e45-a00e-0123926fbc87
• CalendarAvailability: a92c3675-dbc8-44f6-b24e-520eeadb8864

This PR is intentionally a draft. Calex will not write records using these schema IDs until W3DS maintainers approve the model and merge/register the schemas.

Motivation

Calex needs interoperable records for calendar visibility, delegated event operations, explicit availability windows, and public booking policy without introducing a local authorization database or copying authoritative calendar data outside users' eVaults.

Maintainer questions / blockers

1. Is the canonical/reference AccessGrant pattern appropriate for cross-eVault discovery?
2. May a certified post-platform create a non-authoritative reference in a grantee's eVault?
3. Should raw AccessGrant and CalendarAvailability records remain owner-only under the current combined read/write ACL semantics?
4. Are namespaced permissions such as calendar:viewBusy acceptable?
5. Where should delegated actor and authorizing grant provenance live: MetaEnvelope metadata, audit records, or CalendarEvent fields?
6. Does grant creation/revocation require an owner signature flow?
7. What shared provisional-hold and atomic compare-and-create mechanism should prevent double booking?
8. Are draft-07 conditional keywords (if/then, const) supported by the deployed Ontology validation path?

Scope

Only two new files under services/ontology/schemas/ are added. No existing schema, runtime service, workflow, or deployment configuration is changed.

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