[codex] persist learner configs and metric specs

[salim4n]

Closes #53

What changed

• Added exported default/validated configs and getConfig() for baseline learners.
• Added stable learner metric catalogs for baseline and neural adapter metrics.
• Persisted effective learnerConfig into SDK run configs after pre-run validation.
• Documented baseline and neural adapter defaults.

Validation

• bun run build:packages
• bun test packages/learning/test/config-metrics.test.ts packages/sdk/test/experiment.test.ts
• bun run ci