Add `schemas/actor.json` for registry actor entries

[Faux16]

Context

The openagp/registry README documents that each actor entry validates against schemas/actor.json (coming). The schema doesn't exist yet, which means:

• Registry submissions can't be schema-validated end-to-end (only JSON-parse-validated).
• The registry CI workflow at openagp/registry/.github/workflows/validate-entries.yml has a clearly-marked skip guard that auto-activates once this schema lands.

Goal

Publish schemas/actor.json (JSON Schema Draft 2020-12) that defines the shape of a registry actor entry.

Scope

The schema should constrain:

• actor.fqdn — the actor's fully-qualified domain (must match the filename per registry convention).
• actor.kind — enum: vendor | plane | customer.
• actor.conformance_level — enum: L1 | L2 | L3.
• actor.public_keys[] — array of Ed25519 keys, each with key_id, purpose, and the public key bytes per ADR 0001.
• actor.endpoints — URLs the actor exposes (varies by kind).
• actor.conformance_report — reference to the signed CTS conformance report (location + signature).
• Top-level signature over the canonicalized entry (so the registry index can be mirrored and verified independently).

Also add 1–2 example fixtures under fixtures/actors/ and wire validation through the existing validate-fixtures workflow.

Out of scope

• Federation between registries (deferred to v0.2+ per spec §3.7).
• Trust-store distribution mechanism (separate question).

Pointers

• Registry README: openagp/registry/README.md
• Existing schemas for style match: openagp/spec/schemas/
• ADR 0001 (signature canonicalization): openagp/spec/decisions/0001-signature-canonicalization.md

Unblocks

• Full schema validation in openagp/registry CI.
• First real registry submissions.

[Faux16]

Shipped in #14 — schemas/actor.json is merged, the registry CI validates against it, and both SDKs support kind="actor" (released in openagp 0.0.2 / @openagp/sdk 0.0.2). Closing.