p9: add writable s/{id}/env file for session-scoped env injection

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

internal/p9/sessionfilestore.go

@@ -33,6 +33,7 @@ var sessionFileList = []struct {
 	{"mcp", 0444},
 	{"systemprompt", 0444},
 	{"params", 0666},
+	{"env", 0200},
 }
 
 // SessionFileStore implements ReadWriteStore for the files within a single
@@ -152,6 +153,15 @@ func (s *SessionFileStore) Put(name string, data []byte) error {
 		}
 		return s.sess.core.SetGenerationParams(params)
 
+	case "env":
+		for _, line := range strings.Split(string(data), "\n") {
+			k, v, ok := strings.Cut(strings.TrimSpace(line), "=")
+			if !ok || k == "" {
+				continue
+			}
+			s.sess.core.SetEnv(k, v)
+		}
+
 	}
 	return nil
 }