feat: AI based permission checks

migrations/20260730120000_add_session_permission_mode/down.sql

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+ALTER TABLE sessions DROP COLUMN permission_mode;

migrations/20260730120000_add_session_permission_mode/up.sql

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+ALTER TABLE sessions ADD COLUMN permission_mode TEXT NOT NULL DEFAULT 'heuristic'
+    CHECK (permission_mode IN ('heuristic', 'ai'));