Add a plain-JSON export format to `goldset export` (`--format json`)

[AshwinUgale]

Context

export.py and the CLI hardcode promptfoo as the only export target — cli.py uses choices=["promptfoo"], and the module docstring literally says "The first exporter targets promptfoo." Runners that aren't promptfoo currently have no first-class path.

Task

Add a plain-JSON exporter that emits the golden (confirmed) suite as the internal case model — id, input, assertions, expected_output, trust_tier, set_role. Wire it into --format json.

Acceptance criteria

• Same golden gate as promptfoo: fails closed with NothingGoldenError when nothing is confirmed; --include-candidates marks the artifact non-golden.
• Structural assertions (tool_called / final_state) are preserved, not dropped.
• Test added in tests/test_export.py; README "Export targets" section updated.

Pointers

• src/goldset/export.py
• src/goldset/cli.py (the --format choices for the export subcommand)

Notes for contributors

Keep it in the dependency-light core (stdlib json). Follow the existing fail-closed stance in to_promptfoo — a golden export must never silently emit an empty or unconfirmed set.