feat: --sandbox Docker isolation + full unit test suite (36 tests)

Sandbox: Each session runs inside a fresh Docker container with:
- --cap-drop ALL, --security-opt no-new-privileges
- --network none, --tmpfs /tmp:noexec
- Working directory mounted read-only at /workspace
- Container destroyed on agent.Close()

Tests cover:
- kode.go: Config defaults, API key fallback, thinking passthrough (11 tests)
- llm/client.go: JSON marshaling, thinking/reasoning_effort, response parsing (11)
- loop/loop.go: ReAct engine with httptest mock server (7)
- tool/registry.go: CRUD, duplicates, thread safety (7)

Author: molty3000

.gitignore

@@ -1,3 +1,4 @@
 bin/
+/kode
 *.test
 coverage.out

cmd/kode/main.go

@@ -4,6 +4,7 @@ import (
 	"context"
 	"fmt"
 	"os"
+	"os/exec"
 	"os/signal"
 	"strings"
 
@@ -96,20 +97,61 @@ done:
 	ctx, cancel := signal.NotifyContext(context.Background(), os.Interrupt)
 	defer cancel()
 
+	// --- sandbox setup ---
+	var sandboxCleanup func() error
+	tools := builtinTools()
+
+	if sandbox {
+		containerName := fmt.Sprintf("kode-%d", os.Getpid())
+		fmt.Fprintf(os.Stderr, "kode: starting sandbox container %s...\n", containerName)
+
+		wd, err := os.Getwd()
+		if err != nil {
+			fmt.Fprintf(os.Stderr, "kode: sandbox: getwd: %v\n", err)
+			os.Exit(1)
+		}
+
+		createCmd := exec.Command("docker", "run",
+			"--rm",                                  // destroy on exit
+			"--detach",                              // run in background
+			"--name", containerName,
+			"--cap-drop", "ALL",                     // no capabilities
+			"--security-opt", "no-new-privileges",   // no privilege escalation
+			"--network", "none",                     // no network
+			"--tmpfs", "/tmp:noexec",                // no executable temp files
+			"-v", wd+":/workspace:ro",               // working dir read-only
+			"alpine:latest",
+			"sleep", "infinity",
+		)
+		createCmd.Stderr = os.Stderr
+		if err := createCmd.Run(); err != nil {
+			fmt.Fprintf(os.Stderr, "kode: sandbox: failed to create container: %v\n", err)
+			os.Exit(1)
+		}
+
+		sandboxCleanup = func() error {
+			fmt.Fprintf(os.Stderr, "kode: destroying sandbox container %s...\n", containerName)
+			return exec.Command("docker", "rm", "-f", containerName).Run()
+		}
+
+		// Wire the shell tool to execute commands inside the sandbox.
+		tools[0].(*shellTool).containerName = containerName
+	}
+
 	agent, err := kode.New(kode.Config{
-		Model:         model,
-		BaseURL:       baseURL,
-		MaxIterations: maxIter,
-		SystemMessage: system,
-		Thinking:      thinking,
-		Tools:         builtinTools(),
+		Model:          model,
+		BaseURL:        baseURL,
+		MaxIterations:  maxIter,
+		SystemMessage:  system,
+		Thinking:       thinking,
+		Tools:          tools,
+		SandboxCleanup: sandboxCleanup,
 	})
 	if err != nil {
 		fmt.Fprintf(os.Stderr, "kode: %v\n", err)
 		os.Exit(1)
 	}
-
-	_ = sandbox // TODO
+	defer agent.Close()
 
 	modelName := model
 	if modelName == "" {

cmd/kode/shell.go

@@ -9,7 +9,11 @@ import (
 )
 
 // shellTool lets the agent run shell commands.
-type shellTool struct{}
+// When containerName is set, commands execute inside that Docker container
+// via "docker exec". Otherwise they run on the host.
+type shellTool struct {
+	containerName string // empty = host, non-empty = docker exec into this container
+}
 
 func (t *shellTool) Name() string        { return "shell" }
 func (t *shellTool) Description() string { return "Run a shell command and return its output. Use for: reading files, listing directories, running tests, building code, git operations. The command runs in the current working directory." }
@@ -37,7 +41,13 @@ func (t *shellTool) Call(args string) (string, error) {
 		return "", fmt.Errorf("shell: empty command")
 	}
 
-	cmd := exec.Command("sh", "-c", input.Command)
+	var cmd *exec.Cmd
+	if t.containerName != "" {
+		cmd = exec.Command("docker", "exec", t.containerName, "sh", "-c", input.Command)
+	} else {
+		cmd = exec.Command("sh", "-c", input.Command)
+	}
+
 	var outBuf, errBuf bytes.Buffer
 	cmd.Stdout = &outBuf
 	cmd.Stderr = &errBuf