communicate: add Codex app-server adapter

[willwashburn]

Summary

• Add a Codex communicate adapter backed by codex app-server stdio JSON-RPC.
• Add a reusable JSON-RPC client, Codex handle APIs (ready, send, steer, interrupt, fork, close), relay inbox injection on item/completed / turn/completed, relaycast MCP registration, and a minimum Codex version probe.
• Export onCodexRelay, add the framework: 'codex' discriminator in onRelay(), add the package subpath export, and document the PTY-vs-app-server tradeoff.

Fixes #803

Testing

• npm --prefix packages/sdk run check
• npx tsc -p packages/sdk/tsconfig.build.json --noEmit
• npx tsc -p packages/sdk/tsconfig.build.json
• npx vitest run src/__tests__/communicate/adapters/codex.test.ts src/__tests__/communicate/adapters/codex-jsonrpc.test.ts --config vitest.config.ts
• npx vitest run src/__tests__/communicate --config vitest.config.ts
• Real Codex app-server initialize probe against installed codex-cli 0.125.0

Note: npm --prefix packages/sdk run build is currently blocked before SDK compilation in the existing prebuild chain: packages/github-primitive cannot resolve @agent-relay/workflow-types because the workspace package is not linked in node_modules. The direct SDK build-config compile and declaration emit pass.

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