agent: separate uname (principal) from session ID

Session identity is now a numeric UID (starting at 10000) assigned at
creation and never changed. Session names can be renamed freely without
affecting the 9P identity.

- AgentCoreConfig.Uname carries the principal
- pushSessionEnv sets OLLIE_UNAME on the execute server
- startMount gated by WithMount() option (not env var)
- Close() uses context timeout to avoid hanging on fusermount

pkg/agent/core.go

@@ -235,6 +235,7 @@ type AgentCoreConfig struct {
 	AgentsDir     string
 	SessionsDir   string
 	SessionID     string
+	Uname         string // immutable user principal for 9P identity
 	CWD           string // working directory for tool execution and system prompt
 	Session       *Session
 	Env           AgentEnv
@@ -257,6 +258,7 @@ type agent struct {
 	agentsDir       string
 	sessionsDir     string
 	sessionID       string
+	uname           string
 	dispatcher      tools.Dispatcher
 	newDispatcher   func() tools.Dispatcher
 	newBackend      func(string) (backend.Backend, error)
@@ -311,6 +313,9 @@ func (s *agent) pushSessionEnv() {
 	if srv, ok := s.dispatcher.GetServer("execute"); ok {
 		if es, ok := srv.(tools.EnvSetter); ok {
 			es.SetEnv("OLLIE_SESSION_ID", s.sessionID)
+			if s.uname != "" {
+				es.SetEnv("OLLIE_UNAME", s.uname)
+			}
 		}
 	}
 }
@@ -416,6 +421,7 @@ func NewAgentCore(cfg AgentCoreConfig) Core {
 		agentsDir:       cfg.AgentsDir,
 		sessionsDir:     cfg.SessionsDir,
 		sessionID:       cfg.SessionID,
+		uname:           cfg.Uname,
 		cwd:             paths.ExpandHome(cfg.CWD),
 		startupMessages: cfg.Env.Messages,
 		dispatcher:      cfg.Env.dispatcher,

pkg/tools/execute/server.go

@@ -42,8 +42,9 @@ type Server struct {
 	lockDir string
 
 	// mountProc is the ollie-9p-mount subprocess for this session's FUSE proxy.
-	mountProc *os.Process
-	mountPath string
+	mountProc    *os.Process
+	mountPath    string
+	mountEnabled bool
 
 	// Strict rejects inline {code} steps; only {tool} steps are allowed.
 	Strict bool
@@ -75,6 +76,9 @@ func WithStrict() Option { return func(s *Server) { s.Strict = true } }
 // WithYolo skips the landrun sandbox.
 func WithYolo() Option { return func(s *Server) { s.Yolo = true } }
 
+// WithMount enables per-session FUSE mount for 9P identity.
+func WithMount() Option { return func(s *Server) { s.mountEnabled = true } }
+
 // WithAllowExecutors restricts which executors are available.
 func WithAllowExecutors(names []string) Option {
 	return func(s *Server) {
@@ -362,7 +366,8 @@ func (e *Server) SetEnv(key, value string) {
 		e.envExtra = make(map[string]string)
 	}
 	e.envExtra[key] = value
-	if key == "OLLIE_SESSION_ID" && value != "" {
+	// Start mount once we have the principal.
+	if key == "OLLIE_UNAME" && value != "" {
 		e.startMount(value)
 	}
 	e.envMu.Unlock()
@@ -370,9 +375,12 @@ func (e *Server) SetEnv(key, value string) {
 
 // startMount launches ollie-9p-mount for this session. Must be called with envMu held.
 func (e *Server) startMount(sessionID string) {
+	if !e.mountEnabled {
+		return
+	}
 	bin, err := exec.LookPath("ollie-9p-mount")
 	if err != nil {
-		return // binary not installed; skip silently
+		return
 	}
 	mnt := filepath.Join(os.TempDir(), "ollie-mount-"+sessionID)
 	os.MkdirAll(mnt, 0755) //nolint:errcheck
@@ -404,9 +412,11 @@ func (e *Server) Close() {
 	e.mountPath = ""
 	e.envMu.Unlock()
 	if proc != nil {
-		exec.Command("fusermount", "-u", mnt).Run()     //nolint:errcheck
-		syscall.Kill(proc.Pid, syscall.SIGKILL)         //nolint:errcheck
-		os.Remove(mnt)                                  //nolint:errcheck
+		ctx, cancel := context.WithTimeout(context.Background(), 2*time.Second)
+		exec.CommandContext(ctx, "fusermount", "-u", mnt).Run() //nolint:errcheck
+		cancel()
+		syscall.Kill(proc.Pid, syscall.SIGKILL) //nolint:errcheck
+		os.Remove(mnt)                          //nolint:errcheck
 	}
 }