Add SKIP_MIGRATIONS to run migrations out-of-band

[iplay88keys]

Important

Branched off of #2168, so will be rebased after that merges.

Description

Adds a SKIP_MIGRATIONS opt-out so database migrations can run out-of-band (e.g. from a CI/CD pipeline) instead of at controller startup.

• New --skip-migrations controller flag (settable via the SKIP_MIGRATIONS env var; default off — startup migrations remain the default behavior).
• When set, the controller applies nothing and instead calls the new migrations.VerifyMigrated, refusing to start against a database that is not fully migrated. Per source: a missing tracking table, a version behind the binary's embedded max, or a dirty tracking table is a startup error with an actionable message; a database ahead of the binary boots in compatibility mode, matching RunUp.
• VerifyMigrated issues only SELECTs — it deliberately does not open golang-migrate, which creates the tracking table on every open — so the verification works on a connection whose role has no DDL privileges.
• Helm: new database.postgres.skipMigrations value (default false) rendered into the controller configmap as SKIP_MIGRATIONS, plus a NOTES warning when enabled reminding the operator that migrations must be applied out-of-band before install/upgrade.

Testing

Verified on a kind cluster: upgrade with skipMigrations=true boots and logs database schema verified; renaming a tracking table makes the controller exit with tracking table "vector_schema_migrations" does not exist — the database has not been migrated; apply migrations out-of-band or unset SKIP_MIGRATIONS; restoring the table recovers.

Fresh install without migrations returns:

{"level":"error","ts":"2026-07-06T17:37:17Z","logger":"setup","msg":"database migration verification failed","error":"source core: tracking table \"schema_migrations\" does not exist - the database has not been migrated; apply migrations out-of-band or unset SKIP_MIGRATIONS","stacktrace":"github.com/kagent-dev/kagent/go/core/pkg/app.Start\n\t/workspace/core/pkg/app/app.go:482\nmain.main\n\t/workspace/core/cmd/controller/main.go:34\nruntime.main\n\t/usr/lib/go/src/runtime/proc.go:290"}

Controller starts up correctly when migrations are manually run against the db:

❯ ./go/core/bin/kagent-local db migrate up
applied 9 migration(s); schema is up to date