Version semantic diff and impact JSON contracts

[dkharlanau]

What changed

• add explicit output metadata to semantic diff and impact JSON: format, contract version, and schema URI
• preserve existing payload fields at the top level for additive compatibility
• add bundled JSON Schemas for both machine-readable outputs
• add process-code output-schema {semantic-diff,impact} for offline schema retrieval
• validate real diff/impact payloads against the schemas in tests
• smoke-test schema resources from the built wheel
• publish the schemas through GitHub Pages and advertise them in the agent manifest
• document the v1 compatibility policy

Compatibility policy

Output contract 1.0 keeps existing documented fields and their types stable. New optional fields may be added; removing, renaming, or changing an existing documented field requires a new major output contract.

Stack

This PR is intentionally based on #40 because the v1 schemas include the governed analysis section added there. After #40 lands, this branch can be retargeted to main without changing the implementation.

[github-actions]

Process as Code — PR impact

No changed process contracts detected.