execute: per-session FUSE mount for 9P identity

The execute server now starts ollie-9p-mount when OLLIE_SESSION_ID is
set, giving each agent session its own FUSE proxy that attaches to
olliesrv with Uname=session_id. Tools see the full namespace through
$OLLIE but all 9P operations carry the session's identity.

Close() tears down the mount on session destroy.

pkg/agent/core.go

@@ -445,6 +445,14 @@ func NewAgentCore(cfg AgentCoreConfig) Core {
 // Close releases resources for this session, including its tmpdir.
 func (s *agent) Close() {
 	s.log.Debug("Close() session=%q", s.sessionID)
+	if s.dispatcher != nil {
+		if srv, ok := s.dispatcher.GetServer("execute"); ok {
+			if c, ok := srv.(interface{ Close() }); ok {
+				s.log.Debug("Close() calling execute.Close()")
+				c.Close()
+			}
+		}
+	}
 	if s.sessionID != "" {
 		os.RemoveAll(filepath.Join(ollieTmpDir(), s.sessionID)) //nolint:errcheck
 	}

pkg/tools/execute/server.go

@@ -41,6 +41,10 @@ type Server struct {
 	// step dispatch. Set once at session init via SetLockDir; empty disables locking.
 	lockDir string
 
+	// mountProc is the ollie-9p-mount subprocess for this session's FUSE proxy.
+	mountProc *os.Process
+	mountPath string
+
 	// Strict rejects inline {code} steps; only {tool} steps are allowed.
 	Strict bool
 
@@ -358,9 +362,54 @@ func (e *Server) SetEnv(key, value string) {
 		e.envExtra = make(map[string]string)
 	}
 	e.envExtra[key] = value
+	if key == "OLLIE_SESSION_ID" && value != "" {
+		e.startMount(value)
+	}
 	e.envMu.Unlock()
 }
 
+// startMount launches ollie-9p-mount for this session. Must be called with envMu held.
+func (e *Server) startMount(sessionID string) {
+	bin, err := exec.LookPath("ollie-9p-mount")
+	if err != nil {
+		return // binary not installed; skip silently
+	}
+	mnt := filepath.Join(os.TempDir(), "ollie-mount-"+sessionID)
+	os.MkdirAll(mnt, 0755) //nolint:errcheck
+	cmd := exec.Command(bin, "-u", sessionID, mnt)
+	cmd.SysProcAttr = &syscall.SysProcAttr{Setpgid: true}
+	if err := cmd.Start(); err != nil {
+		return
+	}
+	// Reap in background so Wait works later.
+	go cmd.Wait() //nolint:errcheck
+	// Wait for mount to be ready.
+	for i := 0; i < 20; i++ {
+		time.Sleep(50 * time.Millisecond)
+		if _, err := os.Stat(filepath.Join(mnt, "s")); err == nil {
+			break
+		}
+	}
+	e.mountProc = cmd.Process
+	e.mountPath = mnt
+	e.envExtra["OLLIE"] = mnt
+}
+
+// Close tears down the per-session FUSE mount.
+func (e *Server) Close() {
+	e.envMu.Lock()
+	proc := e.mountProc
+	mnt := e.mountPath
+	e.mountProc = nil
+	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
+	}
+}
+
 // executeElevated runs cmd outside the sandbox via x/elevate.
 // Returns (output, error); a non-zero exit code is treated as an error.
 func (e *Server) executeElevated(ctx context.Context, cmd, dir string, timeout int) (string, error) {