Publish and validate a JSON schema for policy configuration

[pilipilisbot]

Context

policy.py validates many fields at load time, but there is no standalone policy.schema.json. A schema would make operator config safer and easier to edit before service startup.

Proposal

Add a JSON Schema for policy.json and a validation command such as gab validate-policy.

Acceptance criteria

• Repository includes policy.schema.json or an equivalent generated schema artifact.
• gab validate-policy --policy path reports unknown fields and invalid values clearly.
• Documentation references the schema and validation command.
• Tests cover valid and invalid policy examples.

Priority

Low.

[giscebot]

Implemented in #228. It publishes the Draft 2020-12 policy schema, adds gab validate-policy --policy PATH with unknown-field/path-aware errors plus semantic validation, documents usage, and adds valid/invalid coverage.

Validation: 23 targeted policy tests pass; the canonical example validates; the built wheel contains the schema. The full suite could not collect on this host because its system Python lacks the test-only fastapi dependency and has no pip/ensurepip.

[giscebot]

Resolved by merged PR #228 (merge commit 5f6b333). The pytest 3.11, pytest 3.12, and dashboard checks all passed. I also re-ran the focused policy suite after merge: 23 tests passed, and gab validate-policy accepts policy.example.json. No follow-up code change is needed for this issue.