v0.7.0 — validating problems

Adds an optional validating-problems workflow before Mindstorming. It evaluates existence, audience, materiality, and mechanism/context independently; prefers direct customer and behavioural evidence; routes evidence expectations by product stage; challenges premises and alternatives; and writes evidence-assessed problem briefs without implying prioritisation or approval. One-pagers can consume those briefs while preserving scope, status, limitations, and recency. Full changes are in the CHANGELOG.