Fix reviewer suggestion confidence for sole experts

[simonfaltum]

Why

The suggest-reviewers GitHub Action had two issues:

1. When a single person was the sole contributor to all changed files, confidence was reported as "low" instead of "high". This happened because compute_confidence treated len(ss) < 2 as insufficient data, when it actually means there's one clear expert.
2. Output files (out.*, output.txt) were scored 0.0 and completely excluded from analysis. Reviewers should still verify output changes make sense, so these files deserve a small (but non-zero) contribution to scoring.

Changes

Before: Sole-author PRs always got "low" confidence. Output files were invisible to the scoring algorithm. PRs with exactly 2 contributors also always got "low" confidence (the >= 3 guards prevented any comparison).

Now:

• len(ss) == 1 (sole contributor) returns "high" confidence
• len(ss) == 2 compares top vs second using the same 2x/1.5x thresholds
• Output files get weight 0.01 / total_files instead of 0.0, contributing signal without dominating scores
• Removed dead if weight == 0.0 guard that could never trigger after the weight change

Test plan

• Verified against PR Fix acceptance/bundle/apps/job_permissions #4857 (4 files, 2 output, pietern sole author): would now produce "high" instead of "low"
• Logic review of all compute_confidence branches for correctness

This pull request was AI-assisted by Isaac.

[github-actions]

Eligible reviewers

Could not determine reviewers from git history. Based on CODEOWNERS, these people or teams could review:

@andrewnester, @anton-107, @denik, @pietern, @shreyas-goenka

_{Suggestions based on git history of 1 changed files (0 scored). See CODEOWNERS for path-specific ownership rules.}