Document and version the --json output schema

[wkentaro]

What to build

Give --json a documented, versioned contract so downstream agents and tools
can depend on it. Real usage shows agents guessing at the shape (a flat list of
hunk objects vs. a nested by-file/by-status structure) and silently failing when
they guess wrong. Pin the schema down:

• Add a schema_version to the JSON envelope.
• Document the exact shape (fields, types, status values) in the README.
• Optionally expose the JSON Schema (e.g. a --json-schema flag) so consumers
  can validate.

Acceptance criteria

• list --json output carries an explicit schema_version.
• The JSON shape is documented in the README with field descriptions.
• The contract is covered by a test that would fail if the shape changes
  unintentionally.

Blocked by

None - can start immediately.