Adds source (NOT NULL, backfilled to 'manual_draft'), linked_ticket_id,
linked_ticket_kind, validated_by_outcome columns. CHECK constraints on
source values and linked_ticket_kind values. walked_path lives on the
new l1_walk_sessions table (Task 6) — NOT on FlowProposal.
Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>
