feat(gate): DB boundary oracle — assert the plan's columns physically exist after db:push (P0b)

[agjs]

Observable-gates plan, P0b — the panel's unanimous #1 control (2-round consultation). Second in the false-green-hardening series after #205 (P0a).

A green db:push proves the command ran, not that Postgres holds the plan's schema. That gap is the #200/#204 class: a swallowed crash exits 0, the DB never migrates, every runtime query 500s (column "…" does not exist) while the gate false-greens. New db-oracle.ts queries information_schema after a successful push and asserts the plan's columns are physically present — the observable end state, not the migration's story.

Contract (panel-adjudicated):

• Expected columns from the PLAN (IEntityAcceptance.fields, which already includes parent-FK fields) + the scaffold set {id,user_id,created_at,updated_at} — never from the model's schema file.
• SUBSET-presence (never fail on extras); presence only (SQL type families too coarse); name matching camel/snake-tolerant (userId≡user_id). belongs to User → the existing user_id.
• Inconclusive reads never block — a read failure right after a successful push is transient infra; final acceptance is the backstop. Only a successful read with a missing plan column reds. Table name identifier-guarded.
• Wired into boringstackCommandStage after the db:push success check; surfaces a db-schema-mismatch error pointing at app.schema.ts, short-circuiting before the gate runs on a stale schema.

Proven: 8 oracle unit tests (subset, camel/snake, inconclusive-non-blocking, unsafe-table, missing-column) + a command-stage wiring test (push exit-0 + missing url → db-schema-mismatch) + live integration against valbuild27's Postgres (real bookmark{title,url} passes; expecting a missing column reds). Full suite 3149/0; typecheck + lint clean. Additive/gate-preserving.