Avoid cross-repository PR shorthand misattribution

[laclance]

Summary

Fix the spurious production lookup seen in private run #13:

Nakagawa-master/nakagawa-theory-archive PR #13406

The source comment explicitly identifies Qwen's external PR:

https://github.com/QwenLM/qwen-code/pull/13406

but also repeats implementation PR #13406. The scanner treated that shorthand as if it belonged to the issue repository and attempted a bogus same-repository PR lookup.

This keeps the existing same-repository shorthand mechanism, but ignores a shorthand when a nearby fully-qualified PR URL with the same number clearly points to a different repository.

Regression coverage

• production-shaped Nakagawa/Qwen PR #13406 echo does not trigger a same-repo lookup
• a distant external PR with the same number does not suppress a genuine same-repo shorthand
• existing same-repo shorthand behavior remains covered

Scope

Only:

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

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