[codex] define neural learner adapter contract

[salim4n]

Closes #50

What changed

• Added a typed neural learner adapter contract derived from EnvironmentSpec.
• Normalized observation/action spaces for DQN/PPO/SAC capability checks.
• Added update cadence, stable metric specs, backend descriptors and JSON checkpoint envelopes.
• Documented the contract in learning README and ADR 0006.

Validation

• bun run --cwd packages/learning typecheck
• bun test packages/learning/test/neural-adapter.test.ts packages/learning/test/type-inference.ts
• bun run ci