fix: custom OPENAI_BASE_URL always wins over Codex model alias detection (#222)

* feat: add --provider CLI flag for multi-provider support

Adds a --provider flag that maps friendly provider names to the
environment variables the codebase uses for provider detection.
No more manual env-var configuration — users can now simply run:

  openclaude --provider openai --model gpt-4o
  openclaude --provider gemini --model gemini-2.0-flash
  openclaude --provider ollama --model llama3.2
  openclaude --provider bedrock
  openclaude --provider vertex

Implementation details:
- providerFlag.ts: core logic — maps provider names to env vars,
  uses ??= so explicit env vars always win over the flag defaults
- providerFlag.test.ts: 18 tests covering all 7 providers,
  error messages, model passthrough, and env-var precedence
- cli.tsx: early fast-path (mirrors --bare pattern) — sets env
  vars before Commander option-building and module constants run
- main.tsx: adds --provider to Commander option chain for --help

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

* fix: custom OPENAI_BASE_URL always wins over Codex model alias detection

When OPENAI_MODEL=gpt-5.4 (or gpt-5.4-mini) and a custom OPENAI_BASE_URL
is set (Azure, OpenRouter, etc), the transport was incorrectly forced to
codex_responses because gpt-5.4 is in CODEX_ALIAS_MODELS. This caused
requests to be sent with Codex auth instead of the user's API key,
resulting in 401 Unauthorized errors.

Fix: only use codex_responses when the base URL is explicitly the Codex
endpoint, OR when no custom base URL is set and the model is a Codex
alias. An explicit OPENAI_BASE_URL always takes priority over model-name
based Codex detection.

Verified locally: gpt-5.4 via OpenRouter now correctly shows
Provider=OpenRouter, Endpoint=https://openrouter.ai/api/v1 instead of
routing to chatgpt.com/backend-api/codex.

Fixes #200, #203

---------

Co-authored-by: Claude Sonnet 4.6 <noreply@anthropic.com>

src/services/api/providerConfig.ts

@@ -291,8 +291,13 @@ export function resolveProviderRequest(options?: {
     process.env.OPENAI_BASE_URL ??
     process.env.OPENAI_API_BASE ??
     undefined
+  // Use Codex transport only when:
+  // - the base URL is explicitly the Codex endpoint, OR
+  // - the model is a Codex alias AND no custom base URL has been set
+  // A custom OPENAI_BASE_URL (e.g. Azure, OpenRouter) always wins over
+  // model-name-based Codex detection to prevent auth failures (#200, #203).
   const transport: ProviderTransport =
-    isCodexAlias(requestedModel) || isCodexBaseUrl(rawBaseUrl)
+    isCodexBaseUrl(rawBaseUrl) || (!rawBaseUrl && isCodexAlias(requestedModel))
       ? 'codex_responses'
       : 'chat_completions'