Add a `schema` subcommand that prints the --json output schema

[willkg]

Add a schema subcommand that prints the published JSON Schema for --json output (schema/json-output/v1.json) to stdout, so a consumer — a script, CI, or an agent in Claude Code — can fetch the contract at runtime instead of reading it out of the repo.

Why

--json (#12) is now a versioned, drift-guarded contract (schema_version: 1, validated in tests against schema/json-output/v1.json). Making the schema self-serve from the binary means an agent can validate markfluence's own output, or generate types, without knowing the repo layout.

Shape

• markfluence schema — print the current (latest) schema.
• Embed the file via go:embed so it ships in the binary and can't drift from the tested copy.
• Consider --schema-version N later if/when a v2 exists; for now there is only v1.

Notes

• Tiny command; the schema file and its conformance tests already exist.
• Pairs well with the offline check command and the Claude skill (Write a Claude skill for driving markfluence #36): the skill can point Claude at markfluence schema to learn the output shape.