perf(swarm): skip redundant completion report backfill

[shua0326]

Summary

• skip CommReadContext backfill when an awaited member already carries its retained completion report
• preserve history backfill for done members whose report field is absent
• add focused coverage for reported, report-less, and running members

Why

The formatter already prefers AwaitedMemberStatus.completion_report over the separately fetched history fallback. Fetching full conversation history for a member that already has a report therefore adds one socket round trip and transfers a full history without changing output. The overhead scales linearly with completed swarm width.

This is intentionally a narrow performance change. It does not alter the protocol or rendered report selection.

Validation

• cargo +1.94.1 test --profile selfdev -p jcode-app-core --lib report_backfill_only_covers_done_members_missing_a_retained_report
• cargo +1.94.1 test --profile selfdev -p jcode-app-core --lib format_awaited_members_includes_completion_reports
• cargo +1.94.1 fmt -p jcode-app-core -- --check
• git diff --check

Closes #1264

[greptile-apps]

Confidence Score: 5/5

Safe to merge: focused regression coverage confirms retained reports skip backfill without removing the fallback for report-less completed members.

What we checked:

• Validated that fetch_awaited_member_reports creates CommReadContext requests only for members_needing_report_backfill.
• Confirmed that the selector accepts completed members without a retained report and excludes both completed members with a report and running members.
• Reproduced the earlier selection behavior and ran the focused regression tests and formatter check successfully.
• Identified decisive code locations: CommReadContext construction at communicate.rs:1704-1709 and the selector at communicate.rs:1732, and validated these findings with tests that cover completed-with-report, completed-without-report, and running-without-report.
• Artifacts containing paired before/after script outputs were attached to support the decisive validation.

Summary

This change avoids redundant conversation-history requests when a completed swarm member already has a retained completion report. It preserves history fallback for completed members without a retained report and excludes running members from report backfill.

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