feat(agent): persist recommendation snapshot ids

[JSONbored]

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

Goal

Add stable snapshot ids to recommendations and action packets so every later explanation can refer to one durable decision artifact.

Requirements

• Assign a deterministic or durably persisted snapshot id when a recommendation is created.
• Thread the snapshot id through API, MCP, and control-panel packet shapes where recommendations are already returned.
• Keep this focused on the id/envelope only; do not implement provenance, counterfactuals, or replay UI here.

Acceptance criteria

• Every recommendation emitted by the supported agent path has a snapshot id.
• Existing callers can continue reading recommendations while new callers can key off the snapshot id.
• Public output exposes only the snapshot id and public-safe envelope fields.
• This is not an autonomous PR bot and not public scoring.

Validation expected

• Unit fixtures for snapshot id creation and packet serialization.
• Public/private boundary coverage for the id envelope.
• npm run test:ci.