feat: support openrouter provider pinning

[HwangJohn]

Summary

• Add OPENWIKI_OPENROUTER_PROVIDER_ONLY to pin OpenRouter routing to one upstream provider or a comma-separated allowlist.
• Pass the configured values to ChatOpenRouter as provider.only.
• Persist and surface the new routing key in OpenWiki env diagnostics, and document the setting in the README.

Fixes #39.

Testing

• corepack pnpm exec prettier --write README.md src/constants.ts src/env.ts src/agent/index.ts test/constants.test.ts
• corepack pnpm exec vitest run test/constants.test.ts test/env.test.ts — passed
• corepack pnpm run typecheck — passed
• corepack pnpm exec vitest run test/constants.test.ts test/env.test.ts test/env-behavior.test.ts — only failed the existing Windows chmod assertion in test/env-behavior.test.ts > writes the env file with 0600 permissions