feat(desktop): add minimal hypothesis builder

[Whiteks1]

Goal

Add a minimal but explicit Hypothesis Builder to the desktop shell.

Why

The desktop already supports review and decision flows. It now needs a clearer entry point for defining work before launch.

Scope

• add a minimal structured builder for a hypothesis / prepared run request
• initial fields can be small, for example:
  • statement
  • strategy or model reference
  • key parameters
  • constraints
• connect the builder to the existing launch flow instead of creating a parallel execution system
• keep the first version narrow and auditable

Acceptance criteria

• the desktop has an explicit entry point for hypothesis -> run
• the builder feeds the current launch workflow
• the screen is useful without requiring a new backend subsystem

Out of scope

• no large experiment manager
• no async job orchestration redesign
• no Quant Pulse integration yet
• no Stepbit-driven hypothesis automation in this slice