Add --json output to graphkeeper query

[anusbutt]

Context

Human-readable query output is useful in terminals, but integrations currently must parse presentation text to consume resolved active claims and provenance.

Scope

Specify and implement opt-in graphkeeper query SUBJECT --json output without changing default output, validation order, active-claim selection, or exit codes.

Use only the existing parsed record model and add no runtime dependency.

Acceptance criteria

• The JSON shape is documented and covered as a public compatibility contract.
• Canonical ID, unique alias, empty, ambiguous, unknown, and superseded cases are tested.
• Validation runs before selection exactly as it does for text output.
• Default text output remains byte-for-byte compatible.