feat(agent): add replayable decision snapshots

[JSONbored]

Parent roadmap: #127
Parent phase: #269

Goal

Make every agent recommendation replayable from a stable, inspectable decision snapshot.

This issue is now a mini-epic. Do not implement the whole snapshot system in one PR; use the focused child issues below.

Child issues

• feat(agent): persist recommendation snapshot ids #281 persist recommendation snapshot ids
• feat(agent): add decision snapshot provenance fields #282 add decision snapshot provenance fields
• test(agent): add public/private snapshot serialization fixtures #283 add public/private snapshot serialization fixtures
• feat(agent): add counterfactual reasons to decision snapshots #284 add counterfactual reasons to decision snapshots
• feat(app): display decision snapshot replay details #285 display decision snapshot replay details

Done when

• Snapshot ids, provenance, sanitizer coverage, counterfactuals, and replay UI are complete or intentionally superseded.
• Public/private snapshot boundaries are covered by tests before snapshot details reach GitHub-facing text.
• MCP, API, and control-panel surfaces can reference snapshots without raw source upload.

Non-goals

• No public reward prediction.
• No wallet, hotkey, raw trust, private scoreability, or private reviewability output.
• No autonomous issue filing, PR creation, comments, labels, closure, or merge behavior.

Validation expected

• Child PRs should include targeted snapshot fixtures plus npm run test:ci.