fix(results): omit missing trace sidecar from metrics

[christso]

Summary

• Bead: av-wrf.1
• Follow-up to PR feat(results): write normalized transcript artifacts #1521 after it was merged before the final review fix could attach to the PR head.
• Stop advertising source_artifacts.trace_path in per-run metrics.json unless a trace sidecar path was explicitly provided. This keeps metrics.json from pointing at trace.json for run bundles that do not write that file.

Notes

• The other two requested review items from PR feat(results): write normalized transcript artifacts #1521 were already present on the merged implementation branch after fast-forward: normalizedToolResult emits tool_use.result for status/duration/output presence, and .agents/verification.md documents the ADR 0008 artifact layout.

Verification

• bun --filter @agentv/core build
• bun test apps/cli/test/commands/eval/artifact-writer.test.ts packages/core/test/evaluation/providers/pi-cli-tool-extraction.test.ts apps/cli/test/commands/results/shared.test.ts (73 pass)
• bun run lint
• bun run typecheck

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[christso]

Closing as superseded by #1526. That PR removed the public trace sidecar surface instead of keeping metrics pointed at an optional trace path, so this one-line follow-up no longer matches the current contract.