feat(agent): add counterfactual reasons to decision snapshots

[JSONbored]

Parent roadmap: #127
Parent phase: #269
Parent epic: #270

Goal

Record concise counterfactual reasons in snapshots so a recommendation can explain why obvious alternatives were not selected.

Requirements

• Add counterfactual reason entries for common alternatives such as wait, close, replace, cleanup-first, or choose another issue.
• Keep reasons deterministic and grounded in available signals.
• Avoid payout claims, public score prediction, or private trust/reviewability language.

Acceptance criteria

• Supported recommendation snapshots include ranked or grouped rejected alternatives.

• Counterfactuals distinguish facts from assumptions.

• Public summaries remain sanitized and advisory.

• This is not autonomous PR creation, closure, labeling, or merging.

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

Validation expected

• Counterfactual fixture tests for at least wait, cleanup-first, and choose-another-issue cases.
• Public sanitizer coverage for counterfactual summaries.
• npm run test:ci.