Respect shared tracker claim overrides

[laclance]

Summary

Fix a production false negative from private run #13 for:

• traefik/traefik#12631

This is a contributor task tracker with multiple independent checklist items. An early contributor claimed two annotations that are now completed. Later, a Traefik MEMBER explicitly clarified:

this is a general issue ... all contributions to the remaining tasks are welcome

The scanner still sees the old claim first and rejects the entire issue as active claim by @ris-tlp, hiding the remaining unclaimed work.

This change lets a trusted maintainer's later shared-work clarification clear earlier exclusive claim evidence.

Safety boundary

• only OWNER/MEMBER/COLLABORATOR comments can clear earlier claims
• wording must explicitly identify a general/tracking issue and welcome contributions to remaining tasks
• untrusted equivalent wording does nothing
• any claim made after the maintainer override still blocks normally
• issue-body ownership, linked PRs, and other competition checks are unchanged

Regression coverage

• old claim + trusted shared-tracker override => no active claim
• untrusted override => old claim remains active
• later claim after trusted override => later claim remains active

Scope

Only:

• opportunity_scout/strategic/competition.py
• tests/strategic/test_competition.py

No scoring, readiness, private-instance, or release changes.