Make `find --all` partial scans explicit for rows and counts

[Widthdom]

Summary

Expose the internal scan cap consistently and prevent a non-authoritative count from looking like complete success.

Current v1.38.0 evidence / reproduction

• find QueryCommandRunner --all --limit 3 --json emits only result rows and does not reveal the 250,000-line scan cap.
• find QueryCommandRunner --all --count --json can return scan_truncated:true and authoritative_count:false with exit 0.

Impact

CI and audit tools can treat a lower bound as a complete repository count.

Suggested direction / acceptance criteria

• Emit terminal scan metadata for both row and count modes.
• Add --require-authoritative or a documented partial-result exit status.
• Report the applied cap, scanned amount, and a recovery/continuation action.

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Dogfooding candidate IDs: Q030, Q031. Verified against cdidx v1.38.0 at commit 2919bfc9b1b83cfa7ae1d964845d0f9ffca002d1.

[Widthdom]

Work started on branch fix-issue4561-4578.