Simplify session lifecycle and MCP cleanup

internal/harness/env.go

@@ -8,6 +8,7 @@ import (
 	"path/filepath"
 	"runtime"
 	"strings"
+	"sync"
 
 	"github.com/hjbdev/patterm/internal/projectkey"
 	"github.com/hjbdev/patterm/internal/trust"
@@ -179,9 +180,18 @@ func defaultPattermBin() (string, error) {
 	if p := os.Getenv("PATTERM_BIN"); p != "" {
 		return p, nil
 	}
-	return buildPattermBinary()
+	defaultBinOnce.Do(func() {
+		defaultBinPath, defaultBinErr = buildPattermBinary()
+	})
+	return defaultBinPath, defaultBinErr
 }
 
+var (
+	defaultBinOnce sync.Once
+	defaultBinPath string
+	defaultBinErr  error
+)
+
 func buildPattermBinary() (string, error) {
 	root, err := repoRoot()
 	if err != nil {