feat: support OPENAI_BASE_URL for the openai provider

[Echo (Pahkho)]

Summary

The openai provider had no baseUrlEnvKey, so a custom base URL could only reach it implicitly (relying on the OpenAI SDK to read process.env.OPENAI_BASE_URL). It was not loaded from ~/.openwiki/.env into the OpenWiki-managed environment, never surfaced in credential diagnostics, and never flowed through resolveProviderBaseUrl like the openai-compatible and anthropic providers do.

Changes

• src/constants.ts: add OPENAI_BASE_URL_ENV_KEY and set it as the openai provider's baseUrlEnvKey. resolveProviderBaseUrl("openai") now honors OPENAI_BASE_URL, and the resolved value is passed to the model client via configuration.baseURL (no behavior change when unset — still falls back to the SDK default).
• src/env.ts: register OPENAI_BASE_URL in MANAGED_ENV_KEYS so it is loaded from ~/.openwiki/.env, persisted in stable order, and shown (non-secret) in credential diagnostics alongside ANTHROPIC_BASE_URL / OPENAI_COMPATIBLE_BASE_URL.
• README.md: document OPENAI_BASE_URL for the openai provider under "Alternative base URLs".

Why

Some OpenAI-compatible gateways only expose the Responses API (/v1/responses) and return 502 for chat-completions requests that carry tools. The openai provider already routes tool calls through the Responses API (useResponsesApi: provider === "openai"), so it works against such gateways — but only if the base URL can be configured. With this change, users can target a Responses-API gateway directly from .env:

OPENWIKI_PROVIDER=openai
OPENAI_API_KEY=your-key
OPENAI_BASE_URL=https://your-gateway.example.com/v1
OPENWIKI_MODEL_ID=your-model-name

Previously the only .env-configurable path was openai-compatible, which routes tool calls through chat completions and fails with 502 on these gateways.

Notes

• OPENAI_BASE_URL was already removed from deprecatedEnvKeys on main, so it is already loaded from .env into process.env. This PR makes the openai provider consume it explicitly and consistently rather than relying on the SDK's implicit env-var read.
• No change when OPENAI_BASE_URL is unset; requiresBaseUrl is not set, so the provider remains optional-default.

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