fix: support model parameter for all backends, auto-inject from settings (#105)

- Add Model field to Config and TaskSpec for per-task model selection
- Parse --model flag and model: metadata in parallel tasks
- Auto-inject model from ~/.claude/settings.json for claude backend in new mode
- Pass --model to claude CLI, -m to gemini CLI, --model to codex CLI
- Preserve --setting-sources "" isolation while reading minimal safe subset
- Add comprehensive tests for model parsing, propagation, and settings injection

Fixes #105

Generated with SWE-Agent.ai

Co-Authored-By: SWE-Agent.ai <noreply@swe-agent.ai>

codeagent-wrapper/executor.go

@@ -744,6 +744,10 @@ func buildCodexArgs(cfg *Config, targetArg string) []string {
 		args = append(args, "--dangerously-bypass-approvals-and-sandbox")
 	}
 
+	if model := strings.TrimSpace(cfg.Model); model != "" {
+		args = append(args, "--model", model)
+	}
+
 	args = append(args, "--skip-git-repo-check")
 
 	if isResume {
@@ -788,6 +792,7 @@ func runCodexTaskWithContext(parentCtx context.Context, taskSpec TaskSpec, backe
 		Task:      taskSpec.Task,
 		SessionID: taskSpec.SessionID,
 		WorkDir:   taskSpec.WorkDir,
+		Model:     taskSpec.Model,
 		Backend:   defaultBackendName,
 	}
 
@@ -816,6 +821,15 @@ func runCodexTaskWithContext(parentCtx context.Context, taskSpec TaskSpec, backe
 		return result
 	}
 
+	var claudeEnv map[string]string
+	if cfg.Backend == "claude" {
+		settings := loadMinimalClaudeSettings()
+		claudeEnv = settings.Env
+		if cfg.Mode != "resume" && strings.TrimSpace(cfg.Model) == "" && settings.Model != "" {
+			cfg.Model = settings.Model
+		}
+	}
+
 	useStdin := taskSpec.UseStdin
 	targetArg := taskSpec.Task
 	if useStdin {
@@ -915,10 +929,8 @@ func runCodexTaskWithContext(parentCtx context.Context, taskSpec TaskSpec, backe
 
 	cmd := newCommandRunner(ctx, commandName, codexArgs...)
 
-	if cfg.Backend == "claude" {
-		if env := loadMinimalEnvSettings(); len(env) > 0 {
-			cmd.SetEnv(env)
-		}
+	if cfg.Backend == "claude" && len(claudeEnv) > 0 {
+		cmd.SetEnv(claudeEnv)
 	}
 
 	// For backends that don't support -C flag (claude, gemini), set working directory via cmd.Dir