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/entrypoints/cli.tsx

@@ -399,6 +399,22 @@ async function main(): Promise<void> {
     process.env.CLAUDE_CODE_SIMPLE = '1';
   }
 
+  // --provider: set provider env vars early, before main module loads.
+  // This mirrors the --bare pattern: env vars must be in place before
+  // Commander option building and module-level constants are evaluated.
+  if (args.includes('--provider')) {
+    const { parseProviderFlag, applyProviderFlag } = await import('../utils/providerFlag.js');
+    const provider = parseProviderFlag(args);
+    if (provider) {
+      const result = applyProviderFlag(provider, args);
+      if (result.error) {
+        // biome-ignore lint/suspicious/noConsole:: intentional error output
+        console.error(`Error: ${result.error}`);
+        process.exit(1);
+      }
+    }
+  }
+
   // No special flags detected, load and run the full CLI
   if (process.env.OPENCLAUDE_ENABLE_EARLY_INPUT === '1') {
     const {