docs: fix stale AGENTS.md claims about migrations, 404 handling, and ty suppressions

[lesnik512]

Audited AGENTS.md against the code after #47. Corrections:

• Migrations: migrations/env.py sets the driver to postgresql+psycopg (psycopg 3, per pyproject.toml), not postgresql / psycopg2.
• Domain exceptions: there is a NotFoundError → not_found_error_handler (404) registered next to DuplicateKeyError (400). No handler raises litestar.exceptions.HTTPException.
• ty suppressions: the only ty: ignore[invalid-argument-type] comments are on LifespanManager / ASGITransport in tests/conftest.py. The "DTO list construction" suppression no longer exists.
• just test example: tests/test_decks.py::test_create doesn't exist; switched to test_post_decks.
• Smoothed a doubled "so" left in the Test isolation paragraph by docs: describe create_savepoint session instead of removed CustomAsyncSession #47.

Verified unchanged: commands/justfile, Python 3.14 and port 8000, request flow, DI provider scopes (modern-di defaults unscoped providers to Scope.APP), persistence, test isolation, settings, routes, schemas, deck response shapes, ruff config, agent-skills section (missing CONTEXT.md / docs/adr/ is intentional per docs/agents/domain.md).