feat(watcher): industry trend watcher feeding market_signals + Slack digest

See commit message. Closes the "manual peer conversation / forum scanning" loop: Madalin's old workflow is now an autonomous service that runs every 4 days, classifies each item with Claude, persists to market_signals, and posts a grouped-by-theme digest to #cmoagent. Live verified: 377 items classified across 11 sources, 15-item digest posted.