Replace GORM + AutoMigrate with golang-migrate + sqlc

[iplay88keys]

Description

Warning

Upgrading from a version earlier than v0.8.0 is not supported. You must upgrade to v0.8.0+ before upgrading to this version.

GORM's AutoMigrate is additive-only, leaves no migration history, and has no rollback path. This PR replaces it with versioned SQL migrations (golang-migrate) and compile-time type-safe query generation (sqlc).

Migration is now handled by an init container rather than the controller on startup.

A new CI check fails if any existing migration file is modified. Migrations are immutable once merged.

What this changes for operators

• Golang-migrate takes over existing tables: existing tables use CREATE TABLE IF NOT EXISTS, so all data is preserved
• Rollback on failure: if a migration fails mid-run, changes are rolled back automatically before the init container exits non-zero
• Migration history: every schema change is now a numbered, named SQL file checked into the repo

Testing

An integration test (TestUpgradeFromGORM) applies the pre-migration schema, seeds realistic data, runs the new migrations, and verifies everything is readable and writable afterward.

Manual upgrade testing was also performed against a live Kind cluster:

• Deployed from main with all default agents running against PostgreSQL
• Seeded data across all tables: sessions, events, tasks, feedback, tools, toolservers, LangGraph checkpoints, vector memories, and CrewAI flow state
• Deployed the currency-converter-agent (LangGraph) and poem-flow-agent (CrewAI Flow) samples to populate lg_checkpoint, lg_checkpoint_write, and crewai_flow_state
• Upgraded to this branch. The migrate init container ran successfully (core migrations applied, vector migrations applied)
• Verified all row counts matched exactly across all 12 tables after upgrade

Schema validation

To validate the migration produces a correct schema in both upgrade and fresh-install paths, pg_dump --schema-only was captured after each scenario and compared:

• Upgrade path: deployed v0.8.0 (GORM) to Kind, captured the baseline schema, then upgraded to this branch and re-dumped. All 12 tables, columns, types, and indexes were preserved exactly; only schema_migrations, vector_schema_migrations, and memory.id DEFAULT gen_random_uuid() were added
• Fresh install path: reset the PVC and ran make helm-install from scratch. The resulting schema was functionally identical to the upgraded schema (column ordering within tables differs only where GORM's struct-field ordering diverged from the migration file, which has no runtime impact)