cold-learn

Read the active experiment's results from FoxReach, run the statistical test, and propose a cold.md diff (or auto-commit if trust earned). Computes interested-reply rate per variant via foxreach inbox categorize-stats, applies a two-proportion z-test, checks guards (bounce rate, not-interested rate), and either writes .cold/proposed-diff.patch or applies it directly. Use 7+ days after /cold send when an experiment…