fix: align reaper scan with reap eligibility

[Bella-Giraffety]

Summary

• make Scan() exclude agent wisps using the same stale-wisp eligibility that Reap() already uses
• add focused regression coverage so the Scan() function body specifically must contain the agent-bead exclusion
• keep the change scoped to scan/reap eligibility alignment only; purge and stale-issue behavior are unchanged

Related Issue

• Fixes gs-mn3

Bug

A live reproduction on hq showed:

• gt reaper scan --db hq --max-age=24h reported 4 reap candidates
• gt reaper reap --db hq --max-age=24h closed 0

The root cause was a predicate mismatch:

• Scan() counted all stale open wisps
• Reap() explicitly excluded issue_type='agent'

So scan could report candidates that reap would never close, especially when the candidates were all agent wisps.

Why this fix

This is the smallest safe fix: keep the existing Reap() behavior and make Scan() use the same eligibility predicate. That makes the operator-facing scan count truthful without widening the change into purge or stale-issue logic.

Testing

Focused command run:

• GOTOOLCHAIN=auto go test ./internal/reaper -run 'TestReap|TestScanExcludesAgentBeads'

Coverage:

• TestScanExcludesAgentBeads now scopes its assertion to the Scan() function body specifically, so it cannot pass just because Reap() already had the agent exclusion.

Review process

This branch went through pre-implementation review and a second review wave against the implementation branch. Incorporated findings covered:

• keeping the change limited to scan/reap eligibility alignment
• ensuring the regression test actually targets Scan() instead of matching the broader file text
• confirming operator-facing scan/reap counts now correspond for agent wisps

[Bella-Giraffety]

Rebuilt this PR on a clean upstream main base and force-pushed it so it now contains only the scan/reap eligibility alignment fix and focused tests.