feat(agent): add scenario input model and assumptions

[JSONbored]

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

Goal

Define the input model for repo-specific scenarios before implementing individual simulations.

Requirements

• Represent scenario type, repo config, issue/PR state, branch state, known facts, assumptions, and unavailable signals.
• Keep the model deterministic and safe for MCP/API use.
• Do not implement ranking or strategy rendering in this issue.

Acceptance criteria

• Scenario inputs can distinguish facts, assumptions, estimates, and missing data.

• Public-safe serialization excludes private scoreability, reward estimates, wallet, hotkey, raw trust, and private reviewability.

• Source-upload behavior remains fail-closed for local branch inputs.

• This is advisory planning only, not public scoring.

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

Validation expected

• Schema/model tests for complete, partial, and unavailable-signal inputs.
• Public/private serialization tests.
• npm run test:ci.