Added the Coaching specification

Internal: added docs/specs/COACHING.md, the version 1 specification for a Coaching area built on Client Portal and its plans: portals for one person; goals with measures, milestones and goal attainment scaling; actions with if-then plans; habits shown as consistency rather than streaks; check-ins; exercises from Forms and wheels; a resource library; sessions booked from private session types with an allowance, prep, private notes, recaps and a session rating; a journal, wins and messages; reminders with quiet hours and one-tap links; the coach's Overview of clients who need attention; programs whose changes a coach applies to chosen engagements; cohorts; sponsors who see only what the client approves; payment plans through Invoicing and program pages; progress reviews, a coaching log and export; and JSON import. Nine decisions are open for the owner. Nothing is built yet.