fix(cli): classify engine conflict errors with correct exit code

Recognize *specgraph.Error in classifyError so a lock conflict surfaces as CONFLICT (exit 2) instead of a generic INTERNAL_ERROR. Add end-to-end tests proving CLI commands and dual servers coexist on the same directory.

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

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

Author: tyeongkim

internal/cli/output.go

@@ -4,10 +4,12 @@ import (
 	"encoding/json"
 	"errors"
 	"fmt"
+	"strings"
 
 	"github.com/spf13/cobra"
 	"github.com/tyeongkim/spec-graph/internal/jsoncontract"
 	"github.com/tyeongkim/spec-graph/internal/model"
+	"github.com/tyeongkim/spec-graph/pkg/specgraph"
 )
 
 // exitError carries a process exit code; a nil err means stdout already holds
@@ -96,7 +98,18 @@ func classifyError(err error) (errorClass, bool) {
 		return errorClass{"RELATION_NOT_FOUND", 1}, true
 	case errors.As(err, &changesetNotFound):
 		return errorClass{"CHANGESET_NOT_FOUND", 1}, true
-	default:
-		return errorClass{"INTERNAL_ERROR", 1}, false
 	}
+
+	var sgErr *specgraph.Error
+	if errors.As(err, &sgErr) {
+		return errorClass{specgraphCode(sgErr.Code), sgErr.ExitCode()}, true
+	}
+
+	return errorClass{"INTERNAL_ERROR", 1}, false
+}
+
+// specgraphCode maps a specgraph.ErrorCode to the uppercase JSON error code
+// string used in CLI output.
+func specgraphCode(code specgraph.ErrorCode) string {
+	return strings.ToUpper(string(code))
 }

internal/cli/serve_coexist_test.go

@@ -0,0 +1,86 @@
+package cli_test
+
+import (
+	"encoding/json"
+	"io"
+	"os/exec"
+	"testing"
+	"time"
+
+	"github.com/tyeongkim/spec-graph/internal/jsoncontract"
+)
+
+// startServe launches `spec-graph serve` as a long-running subprocess with an
+// open stdin pipe, so the server stays alive (and, under the old lifetime-lock
+// model, would hold the project lock) until the returned stop func is called.
+func startServe(t *testing.T, dir, dbFile string) (stdin io.WriteCloser, stop func()) {
+	t.Helper()
+
+	cmd := exec.Command(binaryPath, "--db", dbFile, "serve")
+	cmd.Dir = dir
+
+	in, err := cmd.StdinPipe()
+	if err != nil {
+		t.Fatalf("serve stdin pipe: %v", err)
+	}
+	if err := cmd.Start(); err != nil {
+		t.Fatalf("start serve: %v", err)
+	}
+
+	// Give the server a moment to open the engine and begin serving.
+	time.Sleep(300 * time.Millisecond)
+
+	stop = func() {
+		_ = in.Close()
+		_ = cmd.Wait()
+	}
+	return in, stop
+}
+
+func TestE2E_CLICoexistsWithRunningServer(t *testing.T) {
+	dbFile := initTestProject(t)
+	dir := t.TempDir()
+
+	if r := runCLI(t, dir, "init", "--db", dbFile); r.exitCode != 0 {
+		t.Fatalf("init: exit=%d stderr=%s", r.exitCode, r.stderr)
+	}
+
+	_, stop := startServe(t, dir, dbFile)
+	defer stop()
+
+	r := runCLI(t, dir, "--db", dbFile, "entity", "add", "--type", "requirement", "--id", "REQ-001", "--title", "Concurrent with server")
+	if r.exitCode != 0 {
+		t.Fatalf("entity add while server running: exit=%d stderr=%s", r.exitCode, r.stderr)
+	}
+
+	r = runCLI(t, dir, "--db", dbFile, "entity", "list")
+	if r.exitCode != 0 {
+		t.Fatalf("entity list while server running: exit=%d stderr=%s", r.exitCode, r.stderr)
+	}
+	var list jsoncontract.EntityListResponse
+	if err := json.Unmarshal([]byte(r.stdout), &list); err != nil {
+		t.Fatalf("entity list unmarshal: %v\nraw: %s", err, r.stdout)
+	}
+	if list.Count != 1 {
+		t.Errorf("entity list count=%d while server running; want 1", list.Count)
+	}
+}
+
+func TestE2E_DualServersCoexist(t *testing.T) {
+	dbFile := initTestProject(t)
+	dir := t.TempDir()
+
+	if r := runCLI(t, dir, "init", "--db", dbFile); r.exitCode != 0 {
+		t.Fatalf("init: exit=%d stderr=%s", r.exitCode, r.stderr)
+	}
+
+	_, stop1 := startServe(t, dir, dbFile)
+	defer stop1()
+	_, stop2 := startServe(t, dir, dbFile)
+	defer stop2()
+
+	r := runCLI(t, dir, "--db", dbFile, "entity", "add", "--type", "requirement", "--id", "REQ-001", "--title", "Two servers up")
+	if r.exitCode != 0 {
+		t.Fatalf("entity add with two servers running: exit=%d stderr=%s", r.exitCode, r.stderr)
+	}
+}