The plan-review gate (coming next) needs to tell its decision questions apart
from clarifier questions in the durable ledger. Add a `kind` column to
pending_questions (values 'clarify' | 'plan_decision').
- Fresh DBs: `kind TEXT NOT NULL DEFAULT 'clarify'` (+ CHECK) in the DDL.
- Live ledger: idempotent additive migration (SCHEMA_VERSION 3→4) — a guarded
ALTER (PRAGMA table_info) run from both migrate() and init_db; legacy rows
take the 'clarify' default, never null. (SQLite can't add a CHECK via ALTER,
so the migrated column is NOT NULL DEFAULT only; value constraint is enforced
on fresh DBs by the CHECK and on all writes by the typed helper.)
- ledger.post_question gains a keyword-only `kind="clarify"` (backward
compatible — existing callers unchanged); PendingQuestion.from_row reads it.
Tests: fresh-DB column+default, idempotent init_db, legacy-DB backfill to
'clarify', plan_decision round-trip. 1396 passed.