Teaches l1_walk_sessions a new session_kind='ai_build' for AI-generated
decision-tree walks. FK shape matches adhoc: both flow_id and
flow_proposal_id must be NULL. Drops and recreates the two affected CHECK
constraints (session_kind allowlist + target_consistency). Migration
beca7464b6b4 chains from b3358ba0e48c.
Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>
