init --migrate-only crashes on pre-v36 brains: column "provider_id" does not exist

Summary

Upgrading a brain from <v0.27 to v0.27+ via gbrain init --migrate-only aborts before the v36 migration runs, because PGLITE_SCHEMA_SQL is replayed at startup and creates indexes referencing columns that only the v36 migration adds.

Repro

1. Brain at schema_version = 35, gbrain CLI v0.26.x.
2. git pull to v0.27+ (introduces v36 migration: subagent_provider_neutral_persistence_v0_27, adds subagent_messages.provider_id and schema_version, plus matching columns on subagent_tool_executions).
3. gbrain init --migrate-only

Result

error: column "provider_id" does not exist
   at ... CREATE INDEX … ON subagent_messages (job_id, provider_id)

The startup bootstrap aborts before runMigrations() gets a chance to add the column.

Root cause

src/core/pglite-engine.ts::applyForwardReferenceBootstrap is the existing pattern for reconciling PGLITE_SCHEMA_SQL with not-yet-applied migrations (it ALTERs the legacy tables to add columns the bootstrap SQL expects). When v36 added new provider_id / schema_version columns to subagent_messages and subagent_tool_executions and PGLITE_SCHEMA_SQL was extended to index them (see line ~339 of the schema string), applyForwardReferenceBootstrap was not extended to cover the new columns.

Result: any brain that was created on v0.26 or earlier, then upgraded to v0.27+ before running migrations, hits this on every subsequent init.

Workaround (manual)

ALTER TABLE subagent_messages
  ADD COLUMN IF NOT EXISTS schema_version INTEGER NOT NULL DEFAULT 1,
  ADD COLUMN IF NOT EXISTS provider_id    TEXT;

ALTER TABLE subagent_tool_executions
  ADD COLUMN IF NOT EXISTS schema_version INTEGER NOT NULL DEFAULT 1,
  ADD COLUMN IF NOT EXISTS provider_id    TEXT;

Then gbrain init --migrate-only succeeds and v36 records itself as applied.

Suggested fix

Extend applyForwardReferenceBootstrap to add the v36 columns the same way it handles the older forward refs. Optionally: have the bootstrap derive the required columns from the migration list rather than hard-coding them, so future migrations don't repeat this.

Environment

• gbrain v0.28.3 (also reproduces against any v0.27.x → v0.28.x upgrade from a pre-v36 brain)
• macOS 15 / arm64
• bun 1.x
• PGLite (default engine)