Fix external contributor concurrency to be global across all branches

[ko3n1g]

Summary

• The test-queue approval bot previously enforced MAX_CONCURRENCY_EXTERNAL per branch (main/dev/others), allowing up to 3 external workflows to run simultaneously (one per matrix cell).
• Added matches_contributor() helper to count external workflows across all branches without branch filtering.
• For external contributors, the running/queued count is now global; each matrix cell sees the same total, so only 1 external workflow runs at a time.
• Lowered MAX_CONCURRENCY_EXTERNAL default from 3 to 1 to reflect the global intent.
• Internal contributor logic is unchanged (still per-branch).

Test plan

• Verify that with 1 external PR already running, no additional external PRs are approved regardless of which branch they target
• Verify that internal PRs are unaffected and still respect per-branch concurrency limits

🤖 Generated with Claude Code

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