refactor: restructure codebase to internal/ directory with modular architecture

- Move all source files to internal/{app,backend,config,executor,logger,parser,utils}
- Integrate third-party libraries: zerolog, goccy/go-json, gopsutil, cobra/viper
- Add comprehensive unit tests for utils package (94.3% coverage)
- Add performance benchmarks for string operations
- Fix error display: cleanup warnings no longer pollute Recent Errors
- Add GitHub Actions CI workflow
- Add Makefile for build automation
- Add README documentation

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

codeagent-wrapper/internal/backend/gemini.go

@@ -0,0 +1,110 @@
+package backend
+
+import (
+	"os"
+	"path/filepath"
+	"strings"
+
+	config "codeagent-wrapper/internal/config"
+)
+
+type GeminiBackend struct{}
+
+func (GeminiBackend) Name() string    { return "gemini" }
+func (GeminiBackend) Command() string { return "gemini" }
+func (GeminiBackend) Env(baseURL, apiKey string) map[string]string {
+	baseURL = strings.TrimSpace(baseURL)
+	apiKey = strings.TrimSpace(apiKey)
+	if baseURL == "" && apiKey == "" {
+		return nil
+	}
+	env := make(map[string]string, 2)
+	if baseURL != "" {
+		env["GOOGLE_GEMINI_BASE_URL"] = baseURL
+	}
+	if apiKey != "" {
+		env["GEMINI_API_KEY"] = apiKey
+	}
+	return env
+}
+func (GeminiBackend) BuildArgs(cfg *config.Config, targetArg string) []string {
+	return buildGeminiArgs(cfg, targetArg)
+}
+
+// LoadGeminiEnv loads environment variables from ~/.gemini/.env
+// Supports GEMINI_API_KEY, GEMINI_MODEL, GOOGLE_GEMINI_BASE_URL
+// Also sets GEMINI_API_KEY_AUTH_MECHANISM=bearer for third-party API compatibility
+func LoadGeminiEnv() map[string]string {
+	home, err := os.UserHomeDir()
+	if err != nil || home == "" {
+		return nil
+	}
+
+	envDir := filepath.Clean(filepath.Join(home, ".gemini"))
+	envPath := filepath.Clean(filepath.Join(envDir, ".env"))
+	rel, err := filepath.Rel(envDir, envPath)
+	if err != nil || rel == ".." || strings.HasPrefix(rel, ".."+string(os.PathSeparator)) {
+		return nil
+	}
+
+	data, err := os.ReadFile(envPath) // #nosec G304 -- path is fixed under user home and validated to stay within envDir
+	if err != nil {
+		return nil
+	}
+
+	env := make(map[string]string)
+	for _, line := range strings.Split(string(data), "\n") {
+		line = strings.TrimSpace(line)
+		if line == "" || strings.HasPrefix(line, "#") {
+			continue
+		}
+		idx := strings.IndexByte(line, '=')
+		if idx <= 0 {
+			continue
+		}
+		key := strings.TrimSpace(line[:idx])
+		value := strings.TrimSpace(line[idx+1:])
+		if key != "" && value != "" {
+			env[key] = value
+		}
+	}
+
+	// Set bearer auth mechanism for third-party API compatibility
+	if _, ok := env["GEMINI_API_KEY"]; ok {
+		if _, hasAuth := env["GEMINI_API_KEY_AUTH_MECHANISM"]; !hasAuth {
+			env["GEMINI_API_KEY_AUTH_MECHANISM"] = "bearer"
+		}
+	}
+
+	if len(env) == 0 {
+		return nil
+	}
+	return env
+}
+
+func buildGeminiArgs(cfg *config.Config, targetArg string) []string {
+	if cfg == nil {
+		return nil
+	}
+	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)
+		}
+	}
+
+	// Use positional argument instead of deprecated -p flag.
+	// For stdin mode ("-"), use -p to read from stdin.
+	if targetArg == "-" {
+		args = append(args, "-p", targetArg)
+	} else {
+		args = append(args, targetArg)
+	}
+
+	return args
+}