fix: prioritize ANTHROPIC_MODEL env var over settings.model in model … (#505)

Title:
fix: ANTHROPIC_MODEL env var not respected in ACP agent model selection
                                                                  
Body:
   
## Summary
                                                                  
The `ANTHROPIC_MODEL` environment variable is not being respected when
using Claude Code via the ACP (Agent Client Protocol). This causes users
who set their preferred model via
environment variable to have their setting ignored, with the default
model (`claude-sonnet-4-6`) being used instead.
  ## Problem                                                      

When using Claude Code through ACP integrations (e.g., agent platforms),
users expect the `ANTHROPIC_MODEL` environment variable to control which
model is used for LLM requests.
However, the current implementation only checks `settings.model` (from
configuration files) and completely ignores `ANTHROPIC_MODEL`.
  ## Root Cause                                                   

In `src/acp-agent.ts`, the `getAvailableModels` function only checks
`settings.model`:

  ```typescript
  let currentModel = models[0];

if (settings.model) {
    const match = resolveModelPreference(models, settings.model);
if (match) {
      currentModel = match;                                       
    }
  }

The ANTHROPIC_MODEL environment variable is never read, so
environment-based model configuration is silently ignored.
   
Reproduction
                                                                  
1. Set ANTHROPIC_MODEL=glm-4.7 (or any custom model)
  2. Start Claude Code via ACP protocol
3. Send a prompt request
4. Capture network traffic - the actual model used is claude-sonnet-4-6
instead of glm-4.7
  Fix
Added priority-based model selection that respects the ANTHROPIC_MODEL
environment variable, aligning with the main Claude Code CLI behavior:
   
// Model priority (highest to lowest):
  // 1. ANTHROPIC_MODEL environment variable                      
  // 2. settings.model (user configuration)
// 3. models[0] (default first model)
  if (process.env.ANTHROPIC_MODEL) {
const match = resolveModelPreference(models,
process.env.ANTHROPIC_MODEL);
    if (match) {                                                  
      currentModel = match;
    }
  } else if (settings.model) {
const match = resolveModelPreference(models, settings.model);
    if (match) {
currentModel = match;
    }                                                             
  }

This matches the model selection priority used by the main Claude Code
CLI.
   
Testing
                                                                  
- Verified ANTHROPIC_MODEL is now respected when set
  - Verified settings.model still works when ANTHROPIC_MODEL is not set
- Verified default model (models[0]) is used when neither is set
  Breaking Change
None - this fix makes behavior more consistent with user expectations
and the main CLI.

src/acp-agent.ts

@@ -1699,7 +1699,16 @@ async function getAvailableModels(
 
   let currentModel = models[0];
 
-  if (settings.model) {
+  // Model priority (highest to lowest):
+  // 1. ANTHROPIC_MODEL environment variable
+  // 2. settings.model (user configuration)
+  // 3. models[0] (default first model)
+  if (process.env.ANTHROPIC_MODEL) {
+    const match = resolveModelPreference(models, process.env.ANTHROPIC_MODEL);
+    if (match) {
+      currentModel = match;
+    }
+  } else if (settings.model) {
     const match = resolveModelPreference(models, settings.model);
     if (match) {
       currentModel = match;