Guards against None schemas in _compare_schema_deep, found during
Directus (34K stars) showcase run. Also adds null guards at top
of diff() method.

128 tests pass.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Adds first-class JSON Schema support to delimit-action so it can run
governance checks against schema files like agents-oss/agentspec's
agent.schema.json (issue #21).

Changes:

- core/json_schema_diff.py — new sibling module to diff_engine_v2.
  Handles Draft 4+ single-file schemas with internal $ref resolution
  to #/definitions. Covers 12 v1 change types: property add/remove,
  required add/remove, type widen/narrow, enum add/remove, const,
  additionalProperties, pattern, minLength/maxLength,
  minimum/maximum, items type. Composition keywords (anyOf/oneOf/
  allOf) and external $ref deferred past v1.

- core/generator_drift.py — new module that runs a regen command in
  a sandbox and diffs the regenerated artifact against the committed
  version. Catches drift between source-of-truth (e.g. Zod) and
  committed generated artifacts. Restores the workspace cleanly so
  the working tree is unmodified after the check.

- core/spec_detector.py — added detect_spec_type() classifier and
  get_diff_engine() factory. Routes JSON Schema files to the new
  engine; OpenAPI files keep the existing path. Back-compat: ambiguous
  docs default to OpenAPI.

- action.yml — three new inputs:
  - fail_on_breaking (boolean alias for mode=enforce)
  - generator_command (opt-in drift check)
  - generator_artifact (paired with generator_command)
  Plus dispatch logic in the validate step and a new JSON Schema
  branch in the Comment-on-PR step that renders drift + classification.

- tests/test_json_schema_diff.py — 41 unit tests covering every v1
  change type, $ref resolution at root and nested paths, dispatcher
  routing, and the agentspec rename as a real-world fixture.

Test results:
- 41/41 new JSON Schema tests pass
- 47/47 existing OpenAPI diff_engine tests still pass (no regressions)

Verified end-to-end against the agents-oss/agentspec checkout with
the real pnpm run schema:export generator. All three scenarios
(clean main, simulated rename PR, hypothetical version bump) classify
correctly with expected output.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

The previous header rendered "0 change(s), 1 breaking" when the
committed schema was unchanged but generator drift was detected.
Suppress the schema-classification line entirely when the diff is
empty, and surface drift as a separate "Generator drift: N change(s),
N breaking" line so the two signals never appear comma-joined.

Caught by deliberation on the live infracore/agentspec#1 test run.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>