[codex] Add graph-ai provider registry

[rushillshah]

Closes #12

Scope

• Adds a PROVIDERS registry in lib/graph-ai.js for labels, default models, API key lookup, env help, fallback pricing, and request dispatch.
• Keeps existing OpenAI, Anthropic, xAI, and xci alias behavior while removing repeated provider if chains.
• Extends the existing graph-ai wiring test to assert the registry path.

Validation

• npm run check
• npm test
• node ./bin/digital-brain.js graph-ai --vault ./examples/sample-vault --provider xci --output-tokens 2000

Risk

• Low-to-medium: graph-ai provider metadata is centralized, but no live provider call was made. Estimate-only smoke covered the xci alias path.

[rushillshah]

Maintenance reconciliation note (2026-07-04): CI is passing and the branch is mergeable, but this PR is still draft and review-required. Next action: mark ready/request review before merge.