`gh pr list --head <branch>` matches PRs across all forks, so when a
session's branch is the project default (typically "main") we'd match
unrelated contributors' fork PRs. Two defenses:
1. Short-circuit when session.branch === project.defaultBranch — a worker
session should never legitimately have its head ref on the default
branch, and matching there is unsafe by construction.
2. Request `headRepositoryOwner` from gh and discard PRs whose head owner
differs from the project repo owner. Fork PRs are someone else's work.
Refs #1724.
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