feat: implement Phase 2 (Go Agent) and Phase 3 (React Frontend MVP)

Phase 2 - Go Agent Core:
- gRPC client with exponential backoff reconnect logic
- Command executor (PowerShell/sh cross-platform)
- Proto stubs regenerated with module= option (correct output path)
- gRPC upgraded to v1.79.3 (BidiStreamingClient support)

Phase 3 - React Frontend MVP:
- Vite + React 18 + TypeScript setup with Tailwind CSS v4
- TanStack Query for data fetching, API client + TypeScript types
- Dashboard page: stats cards (agents/status/tickets) + sortable agents table
- Agent detail page: CPU/RAM charts (Recharts), disk usage, shell command executor
- Tickets page: CRUD with modals, filters, sortable table
- Dark mode with CSS custom properties

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

Agent/internal/executor/executor.go

@@ -0,0 +1,49 @@
+package executor
+
+import (
+	"bytes"
+	"context"
+	"os/exec"
+	"runtime"
+	"time"
+)
+
+type Result struct {
+	ExitCode int
+	Stdout   string
+	Stderr   string
+	Success  bool
+}
+
+func Execute(ctx context.Context, command string, timeoutSec int) *Result {
+	if timeoutSec <= 0 {
+		timeoutSec = 300
+	}
+	ctx, cancel := context.WithTimeout(ctx, time.Duration(timeoutSec)*time.Second)
+	defer cancel()
+
+	var cmd *exec.Cmd
+	if runtime.GOOS == "windows" {
+		cmd = exec.CommandContext(ctx, "powershell", "-NoProfile", "-NonInteractive", "-Command", command)
+	} else {
+		cmd = exec.CommandContext(ctx, "/bin/sh", "-c", command)
+	}
+
+	var stdout, stderr bytes.Buffer
+	cmd.Stdout = &stdout
+	cmd.Stderr = &stderr
+
+	err := cmd.Run()
+	result := &Result{
+		Stdout:  stdout.String(),
+		Stderr:  stderr.String(),
+		Success: err == nil,
+	}
+	if cmd.ProcessState != nil {
+		result.ExitCode = cmd.ProcessState.ExitCode()
+	}
+	if err != nil && result.ExitCode == 0 {
+		result.ExitCode = -1
+	}
+	return result
+}