feat(scoring): simulate open-pr pressure scenarios

[JSONbored]

Parent roadmap: #127
Parent phase: #269
Parent epic: #271

Goal

Model how opening another PR affects repo queue pressure and contributor strategy.

Requirements

• Simulate open-new-work, wait, and cleanup-first options using repo queue and maintainer-lane signals.
• Explain likely blockers and tradeoffs without payout claims.
• Keep this scoped to open PR pressure; linked issue eligibility and duplicate/stale blockers are separate child issues.

Acceptance criteria

• Scenario output ranks or compares opening another PR against waiting/cleanup-first.
• Output distinguishes known facts from assumptions.
• Maintainer-lane repos are handled separately from normal contributor-lane scenarios.
• This is not an autonomous PR bot and not public scoring.

Validation expected

• Fixtures for low-pressure, high-pressure, maintainer-lane, and missing-signal repos.
• Public sanitizer tests for scenario summaries.
• npm run test:ci.