refactor(sig-events): remove verdicts data stream, merge audit fields into events

[crespocarlos]

Summary

Collapses the .significant_events-verdicts data stream into .significant_events-events. Audit fields (verdict_summary, assessment_note, verdict_source, original_verdict, conversation_id, grouped_discovery_ids) are merged into sigEventSchema and eventsMappings.

The Verdicts tab and use_fetch_verdicts hook are removed from streams_app; lifecycle timeline and flyout now surface audit information from event documents only.

How to test

• Verify the Significant Events tab loads without errors and the Verdicts tab is absent.
• Open the event lifecycle flyout for a reviewed discovery; confirm verdict_summary and assessment_note appear on the event row when present, with no verdicts section.
• Confirm GET /internal/streams/{name}/sig_events/events and GET /internal/streams/{name}/sig_events/events/{id}/lifecycle respond successfully; lifecycle response body contains { detections, discoveries, events } only (no verdicts key).
• Automated: type-check via node scripts/type_check for kbn-streams-schema, streams, and streams_app.

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🤖 Co-authored with AI assistance.

[infra-vault-gh-plugin-prod]

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