feat: support custom select values in review queue

[skylarbpayne]

Summary

• Adds schema-derived custom select descriptors to Review Queue selection surfaces for scalar and structured typed returns.
• Adds dashboard UI for Other / custom scalar and structured-object input without changing generated option payloads.
• Strips the browser routing marker before engine validation and stamps trusted server-side selection_source: human_custom provenance.
• Coerces {value: ...} select responses for scalar typed returns.

Verification

• pytest tests/test_dashboard_plugin.py -k 'select or selection or review_request' -vv — 5 passed\n- pytest tests/test_authoring_api.py -k 'select or operator_response or schema' -vv — 12 passed\n- pytest tests/test_dashboard_plugin.py tests/test_authoring_api.py -vv — 114 passed\n- git diff --check — passed\n\nCloses Allow custom human-entered values in select() Review Queue cards #128