[codex] add DQN debug learner

[salim4n]

Closes #51

What changed

• Added a TypeScript DqnLearner with replay buffer, epsilon decay, target network and JSON neural checkpoint envelopes.
• Added trainDqn(), DQN config/default exports and stable DQN metrics.
• Enabled DQN checkpoint inference for discrete CLI environments.
• Added CI smoke coverage proving GridWorld DQN inference improves over random.

Validation

• bun run typecheck
• bun test packages/learning/test/dqn.test.ts packages/learning/test/config-metrics.test.ts packages/learning/test/neural-adapter.test.ts packages/cli/test/cli.test.ts
• bun run build:packages && bun scripts/ci-smoke.mjs
• bun run ci