feat: add executable deck brief gate

[1beenlee]

Summary

• add tools/validate-brief.mjs, a zero-dependency validator for the documented DECK_BRIEF.md top-level scalar/list subset
• validate required fields, list/scalar shapes, confidence enum, duplicate/malformed syntax, optional numeric/list fields, and recommended direction
• mechanically reject confidence: high while unresolved missing_information remains
• integrate the brief validator into tools/prepare-deck.mjs; invalid briefs remove stale brief-to-deck-packet.md and cannot advance to generation
• add positive/negative regression coverage over the unchanged fictional public example
• update schema/workflow/agent/product docs and CI while keeping examples, prompts, templates, and skill copies unchanged

Evidence boundary

This gate validates brief structure and a mechanical source-sufficiency invariant. It does not prove that key_points are semantically supported by source material; factual/source review remains separate.

Scope guardrails

• no changes under examples/, templates/, prompts/, .agents/, .claude/
• no YAML/npm dependency, model/provider API, network request, deploy, Preview, or hosted product work

Validation target

node --check tools/validate-brief.mjs
node --check tools/test-brief-validation.mjs
node --check tools/prepare-deck.mjs
node tools/test-brief-validation.mjs
node tools/test-agent-workflow.mjs
node tools/test-editability-eval.mjs
node tools/browser-qa.mjs examples/hackathon-demo/index.html
node tools/test-generation-harness.mjs
node tools/validate-deck.mjs examples/hackathon-demo/index.html

Draft until exact-head CI and diff review are green.