Reject withdrawn implementations awaiting maintainer decision

[laclance]

Summary

Fix the production false positive from etcd-io/etcd#22500 without changing ordinary failed-PR availability.

The reporter originally supplied PR #22501, then closed that implementation after end-to-end benchmarks showed no clear benefit and explicitly left the issue open for maintainers to decide. The scanner still treated the issue as implementation-ready.

This extends the existing reporter-design readiness policy with a narrow conjunctive state:

• the issue reporter says they closed/withdrew their own PR/implementation attempt; and
• the same reporter status explicitly says the issue is now left for maintainers to decide.

A later explicit reporter or trusted-maintainer readiness signal clears the hold.

Regression coverage

• etcd #22500 wording rejects with:
  issue reporter withdrew implementation pending maintainer decision
• a closed/failed PR alone remains actionable
• "maintainers to decide" without reporter withdrawal remains actionable
• the same wording from a non-reporter does not block
• a later trusted maintainer readiness signal restores actionability

Scope

Only:

• opportunity_scout/strategic/readiness.py
• tests/strategic/test_readiness.py

No private-instance changes and no release/promotion changes.