feat: support baseURL config setting (Feat #516)

[nvp159]

Solves #516
### Summary

This PR adds first‐class support for overriding the API base URL used by each provider. You can now:

    * Override the default endpoint via environment variables
        * `OPENAI_BASE_URL` for OpenAI

        * `<PROVIDER>_BASE_URL` (e.g. `OLLAMA_BASE_URL`, `MISTRAL_BASE_URL`, etc.) for other providers
    * Override the endpoint per‐run with the new `--base-url` CLI flag

### Changes

    * **config.ts**
        * Enhanced `getBaseUrl(provider)` to:

            * Respect `OPENAI_BASE_URL` for OpenAI


            * Respect `<PREFIX>_BASE_URL` for all other providers (prefix derived from each provider’s env key)


            * Fall back to the built‑in `providers[provider].baseURL` default
    * **cli.tsx**
        * Introduced a new `--base-url <url>` flag (with help text)

        * When passed, mutate the provider’s `baseURL` in memory before any requests
    * **providers.ts**
        * Defines default `baseURL` and `envKey` for every supported provider

### Usage

    # 1) Environment‐level override (persistent)
    export OPENAI_BASE_URL="https://my-proxy.local/v1"
    codex --provider openai ...

    # 2) One‑off override via CLI
    codex --provider ollama \
          --base-url "http://localhost:11434/v1" \
          "Describe the architecture of …"

[tibo-openai]

Looks like there are other changes from #528 bundled in this PR, could you look at reverting those changes and keep this PR scoped to only the changes necessary to add _BASE_URL? Thanks!