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/backend.go

@@ -4,6 +4,7 @@ import (
 	"encoding/json"
 	"os"
 	"path/filepath"
+	"strings"
 )
 
 // Backend defines the contract for invoking different AI CLI backends.
@@ -37,33 +38,48 @@ func (ClaudeBackend) BuildArgs(cfg *Config, targetArg string) []string {
 
 const maxClaudeSettingsBytes = 1 << 20 // 1MB
 
-// loadMinimalEnvSettings 从 ~/.claude/settings.json 只提取 env 配置。
-// 只接受字符串类型的值；文件缺失/解析失败/超限都返回空。
-func loadMinimalEnvSettings() map[string]string {
+type minimalClaudeSettings struct {
+	Env   map[string]string
+	Model string
+}
+
+// loadMinimalClaudeSettings 从 ~/.claude/settings.json 只提取安全的最小子集：
+// - env: 只接受字符串类型的值
+// - model: 只接受字符串类型的值
+// 文件缺失/解析失败/超限都返回空。
+func loadMinimalClaudeSettings() minimalClaudeSettings {
 	home, err := os.UserHomeDir()
 	if err != nil || home == "" {
-		return nil
+		return minimalClaudeSettings{}
 	}
 
 	settingPath := filepath.Join(home, ".claude", "settings.json")
 	info, err := os.Stat(settingPath)
 	if err != nil || info.Size() > maxClaudeSettingsBytes {
-		return nil
+		return minimalClaudeSettings{}
 	}
 
 	data, err := os.ReadFile(settingPath)
 	if err != nil {
-		return nil
+		return minimalClaudeSettings{}
 	}
 
 	var cfg struct {
-		Env map[string]any `json:"env"`
+		Env   map[string]any `json:"env"`
+		Model any            `json:"model"`
 	}
 	if err := json.Unmarshal(data, &cfg); err != nil {
-		return nil
+		return minimalClaudeSettings{}
 	}
+
+	out := minimalClaudeSettings{}
+
+	if model, ok := cfg.Model.(string); ok {
+		out.Model = strings.TrimSpace(model)
+	}
+
 	if len(cfg.Env) == 0 {
-		return nil
+		return out
 	}
 
 	env := make(map[string]string, len(cfg.Env))
@@ -75,9 +91,19 @@ func loadMinimalEnvSettings() map[string]string {
 		env[k] = s
 	}
 	if len(env) == 0 {
+		return out
+	}
+	out.Env = env
+	return out
+}
+
+// loadMinimalEnvSettings is kept for backwards tests; prefer loadMinimalClaudeSettings.
+func loadMinimalEnvSettings() map[string]string {
+	settings := loadMinimalClaudeSettings()
+	if len(settings.Env) == 0 {
 		return nil
 	}
-	return env
+	return settings.Env
 }
 
 func buildClaudeArgs(cfg *Config, targetArg string) []string {
@@ -93,6 +119,10 @@ func buildClaudeArgs(cfg *Config, targetArg string) []string {
 	// This ensures a clean execution environment without CLAUDE.md or skills that would trigger codeagent
 	args = append(args, "--setting-sources", "")
 
+	if model := strings.TrimSpace(cfg.Model); model != "" {
+		args = append(args, "--model", model)
+	}
+
 	if cfg.Mode == "resume" {
 		if cfg.SessionID != "" {
 			// Claude CLI uses -r <session_id> for resume.
@@ -122,6 +152,10 @@ func buildGeminiArgs(cfg *Config, targetArg string) []string {
 	}
 	args := []string{"-o", "stream-json", "-y"}
 
+	if model := strings.TrimSpace(cfg.Model); model != "" {
+		args = append(args, "-m", model)
+	}
+
 	if cfg.Mode == "resume" {
 		if cfg.SessionID != "" {
 			args = append(args, "-r", cfg.SessionID)