Preserve metadata after conflicted cherry-picks

[svarlamov]

Fixes #1385

Summary

• Recover a single explicit cherry-pick source even when async trace processing runs after --continue removes live sequencer state.
• Preserve source authorship and session metadata so tool_model_breakdown remains populated.
• Keep the fallback bounded to one batched object lookup outside the critical ingestion path.

Test coverage

• Added delayed trace2 conflict/continue coverage with a short source SHA.
• Added stats assertions to the standard conflicted cherry-pick integration test.
• Ran task test, task lint, and task fmt.

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[svarlamov]

• Preserve metadata after conflicted cherry-picks #1911 👈 (View in Graphite)
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[devin-ai-integration]

✅ Devin Review: No Issues Found

Devin Review analyzed this PR and found no potential bugs to report.

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