feat(jwt): add HS256 JWT signing with stdlib-only implementation

Ultraworked with [Sisyphus](https://github.com/code-yeongyu/oh-my-openagent)

Co-authored-by: Sisyphus <clio-agent@sisyphuslabs.ai>

internal/slurm/jwt_test.go

@@ -0,0 +1,169 @@
+package slurm
+
+import (
+	"crypto/hmac"
+	"crypto/sha256"
+	"encoding/base64"
+	"encoding/json"
+	"errors"
+	"os"
+	"strings"
+	"testing"
+	"time"
+)
+
+func TestSignJWT_ValidKey_ProducesValidToken(t *testing.T) {
+	key := []byte("0123456789abcdef0123456789abcdef") // 32 bytes
+	username := "testuser"
+	lifespan := 1 * time.Hour
+
+	token, err := SignJWT(key, username, lifespan)
+	if err != nil {
+		t.Fatalf("SignJWT returned error: %v", err)
+	}
+
+	parts := strings.Split(token, ".")
+	if len(parts) != 3 {
+		t.Fatalf("expected 3 parts, got %d", len(parts))
+	}
+
+	// Decode header
+	headerBytes, err := base64.RawURLEncoding.DecodeString(parts[0])
+	if err != nil {
+		t.Fatalf("decode header: %v", err)
+	}
+	var header map[string]string
+	if err := json.Unmarshal(headerBytes, &header); err != nil {
+		t.Fatalf("unmarshal header: %v", err)
+	}
+	if header["alg"] != "HS256" || header["typ"] != "JWT" {
+		t.Errorf("header = %v, want alg=HS256 typ=JWT", header)
+	}
+
+	// Decode payload
+	payloadBytes, err := base64.RawURLEncoding.DecodeString(parts[1])
+	if err != nil {
+		t.Fatalf("decode payload: %v", err)
+	}
+	var claims JWTClaims
+	if err := json.Unmarshal(payloadBytes, &claims); err != nil {
+		t.Fatalf("unmarshal payload: %v", err)
+	}
+	if claims.Sun != username {
+		t.Errorf("sun = %q, want %q", claims.Sun, username)
+	}
+	if claims.EXP-claims.IAT != 3600 {
+		t.Errorf("exp - iat = %d, want 3600", claims.EXP-claims.IAT)
+	}
+
+	// Verify signature
+	signingInput := parts[0] + "." + parts[1]
+	mac := hmac.New(sha256.New, key)
+	mac.Write([]byte(signingInput))
+	expectedSig := base64.RawURLEncoding.EncodeToString(mac.Sum(nil))
+	if parts[2] != expectedSig {
+		t.Errorf("signature mismatch: got %q, want %q", parts[2], expectedSig)
+	}
+}
+
+func TestSignJWT_UsesRawURLEncoding(t *testing.T) {
+	key := []byte("0123456789abcdef0123456789abcdef")
+	token, err := SignJWT(key, "testuser", 1*time.Hour)
+	if err != nil {
+		t.Fatalf("SignJWT returned error: %v", err)
+	}
+
+	for i, part := range strings.Split(token, ".") {
+		if strings.Contains(part, "=") {
+			t.Errorf("part %d contains padding '=': %q", i, part)
+		}
+	}
+}
+
+func TestSignJWT_ExpiredToken(t *testing.T) {
+	key := []byte("0123456789abcdef0123456789abcdef")
+	token, err := SignJWT(key, "testuser", -1*time.Hour)
+	if err != nil {
+		t.Fatalf("SignJWT returned error: %v", err)
+	}
+
+	parts := strings.Split(token, ".")
+	payloadBytes, err := base64.RawURLEncoding.DecodeString(parts[1])
+	if err != nil {
+		t.Fatalf("decode payload: %v", err)
+	}
+	var claims JWTClaims
+	if err := json.Unmarshal(payloadBytes, &claims); err != nil {
+		t.Fatalf("unmarshal payload: %v", err)
+	}
+
+	now := time.Now().Unix()
+	if claims.EXP >= now {
+		t.Errorf("exp = %d, expected < now (%d)", claims.EXP, now)
+	}
+}
+
+func TestSignJWT_EmptyUsername_Error(t *testing.T) {
+	key := []byte("0123456789abcdef0123456789abcdef")
+	_, err := SignJWT(key, "", 1*time.Hour)
+	if err == nil {
+		t.Fatal("expected error for empty username, got nil")
+	}
+}
+
+func TestReadJWTKey_ValidFile(t *testing.T) {
+	keyData := make([]byte, 32)
+	for i := range keyData {
+		keyData[i] = byte(i)
+	}
+
+	f, err := os.CreateTemp("", "jwtkey-*")
+	if err != nil {
+		t.Fatal(err)
+	}
+	defer os.Remove(f.Name())
+
+	if _, err := f.Write(keyData); err != nil {
+		t.Fatal(err)
+	}
+	f.Close()
+
+	got, err := ReadJWTKey(f.Name())
+	if err != nil {
+		t.Fatalf("ReadJWTKey returned error: %v", err)
+	}
+	if string(got) != string(keyData) {
+		t.Errorf("key mismatch: got %d bytes, want %d bytes", len(got), len(keyData))
+	}
+}
+
+func TestReadJWTKey_FileNotFound(t *testing.T) {
+	_, err := ReadJWTKey("/nonexistent/path/keyfile")
+	if err == nil {
+		t.Fatal("expected error for nonexistent file, got nil")
+	}
+	if !errors.Is(err, os.ErrNotExist) {
+		t.Errorf("error = %v, want os.ErrNotExist", err)
+	}
+}
+
+func TestReadJWTKey_InvalidSize(t *testing.T) {
+	f, err := os.CreateTemp("", "jwtkey-*")
+	if err != nil {
+		t.Fatal(err)
+	}
+	defer os.Remove(f.Name())
+
+	if _, err := f.Write([]byte("short")); err != nil {
+		t.Fatal(err)
+	}
+	f.Close()
+
+	_, err = ReadJWTKey(f.Name())
+	if err == nil {
+		t.Fatal("expected error for short key, got nil")
+	}
+	if !strings.Contains(err.Error(), "key must be") {
+		t.Errorf("error = %q, want message containing 'key must be'", err.Error())
+	}
+}