feat(schemas): open each proposal with a short review focus

Problem

A medium-sized change produces a proposal, design, delta specs and tasks. Reviewers don't know where to look first, so they either read everything or skim everything, and neither catches the places where the agent guessed.

Proposal

Add a short section near the top of schemas/spec-driven/templates/proposal.md, right after ## Why:

## Review Focus
<!-- 3-5 bullets. Leave empty rather than pad. -->
- Decisions a human should confirm
- Assumptions made without asking
- What is deliberately out of scope

Add a matching line to the proposal instruction in schemas/spec-driven/schema.yaml: keep it short, and only list things a reviewer would actually want to check.

Why it fits

It's a template and guidance change only. Artifacts, validation and workflows don't change. Reviewers get a two-minute starting point, so review takes less time and the artifacts keep all their content.