feat(signals): include duplicate and stale-work scenario blockers

[JSONbored]

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

Goal

Add duplicate-risk and stale-work blockers to scenario planning so contributors avoid opening low-value or conflicting work.

Requirements

• Surface duplicate issue/PR signals and stale branch/work signals in scenario outputs.
• Keep evidence references public-safe and concise.
• Do not implement full scenario ranking in this issue unless the ranking surface already exists.

Acceptance criteria

• Scenario blockers include duplicate and stale-work reasons when detected.

• Output includes enough evidence to act without leaking private context.

• Recommendations remain advisory and human-approved.

• This is not public scoring or reward prediction.

• This is not an autonomous PR bot and not public scoring.

Validation expected

• Duplicate, stale branch, stale PR, and no-blocker fixtures.
• Public sanitizer tests for evidence summaries.
• npm run test:ci.