feat: implement 9P mount logic using core execute hooks

Created session/mount.go with all 9P FUSE mount logic previously in core.

Changes:
- New session/mount.go with MountPathFor(), WithMount9P(), and mount lifecycle functions
- WithMount9P() returns execute.Option that wires up OnEnvSet, OnPreDispatch, OnClose hooks
- Updated session.go to use WithMount9P() and MountPathFor() instead of execute.WithMount() and execute.MountPath()
- Mount logic encapsulated in mountState struct with proper synchronization

The mount implementation is now fully contained in the 9p module.

session/mount.go

@@ -0,0 +1,122 @@
+package session
+
+import (
+	"context"
+	"os"
+	"os/exec"
+	"path/filepath"
+	"sync"
+	"syscall"
+	"time"
+
+	"ollie/pkg/tools/execute"
+)
+
+// mountState holds the per-session mount process state.
+type mountState struct {
+	mu       sync.RWMutex
+	proc     *os.Process
+	path     string
+	envExtra map[string]string // reference to Server's envExtra for OLLIE_UNAME lookup
+}
+
+// MountPathFor returns the per-session FUSE mount path for a given uname.
+func MountPathFor(uname string) string {
+	return filepath.Join(os.TempDir(), "ollie-mount-"+uname)
+}
+
+// WithMount9P returns an execute.Option that enables per-session 9P FUSE mounts.
+// It wires up hooks to start, check, and tear down the mount lifecycle.
+func WithMount9P() execute.Option {
+	state := &mountState{envExtra: make(map[string]string)}
+
+	return func(s *execute.Server) {
+		// Hook: OnEnvSet - start mount when OLLIE_UNAME is set
+		s.OnEnvSet = func(key, value string) {
+			state.mu.Lock()
+			state.envExtra[key] = value
+			if key == "OLLIE_UNAME" && value != "" {
+				startMount(state, value)
+			}
+			state.mu.Unlock()
+		}
+
+		// Hook: OnPreDispatch - check mount health before each dispatch
+		s.OnPreDispatch = func() {
+			remountIfStale(state)
+		}
+
+		// Hook: OnClose - tear down mount on session end
+		s.OnClose = func() {
+			teardownMount(state)
+		}
+	}
+}
+
+// startMount launches ollie-9p-mount for this session.
+func startMount(state *mountState, uname string) {
+	bin, err := exec.LookPath("ollie-9p-mount")
+	if err != nil {
+		return
+	}
+	mnt := MountPathFor(uname)
+	os.MkdirAll(mnt, 0755) //nolint:errcheck
+	cmd := exec.Command(bin, "-u", uname, 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
+		}
+	}
+	state.proc = cmd.Process
+	state.path = mnt
+	state.envExtra["OLLIE"] = mnt
+}
+
+// remountIfStale checks the per-session FUSE mount health and re-establishes it if broken.
+func remountIfStale(state *mountState) {
+	state.mu.RLock()
+	mnt := state.path
+	proc := state.proc
+	state.mu.RUnlock()
+	if mnt == "" || proc == nil {
+		return
+	}
+	if _, err := os.Stat(filepath.Join(mnt, "s")); err == nil {
+		return
+	}
+	// Mount is stale — tear down and restart.
+	state.mu.Lock()
+	defer state.mu.Unlock()
+	exec.Command("fusermount", "-u", state.path).Run() //nolint:errcheck
+	syscall.Kill(state.proc.Pid, syscall.SIGKILL)       //nolint:errcheck
+	state.proc = nil
+	state.path = ""
+	if uname := state.envExtra["OLLIE_UNAME"]; uname != "" {
+		startMount(state, uname)
+	}
+}
+
+// teardownMount tears down the per-session FUSE mount.
+func teardownMount(state *mountState) {
+	state.mu.Lock()
+	proc := state.proc
+	mnt := state.path
+	state.proc = nil
+	state.path = ""
+	state.mu.Unlock()
+	if proc != nil {
+		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
+	}
+}

session/session.go

@@ -622,9 +622,9 @@ func (s *Manager) createSession(args []string) error {
 		}
 
 		var execOpts []execute.Option
-		if !s.cfg.NoMount {
-			execOpts = append(execOpts, execute.WithMount())
-		}
+			if !s.cfg.NoMount {
+				execOpts = append(execOpts, WithMount9P())
+			}
 		if s.cfg.Strict {
 			execOpts = append(execOpts, execute.WithStrict())
 		}
@@ -641,7 +641,7 @@ func (s *Manager) createSession(args []string) error {
 			"execute": execute.Decl(cwd, execOpts...),
 		})
 
-		rt := agent.BuildRuntime(cfg, newDisp(), cwd, []string{"OLLIE_SESSION_ID=" + sessID, "OLLIE_UNAME=" + uname, "OLLIE=" + execute.MountPath(uname)})
+		rt := agent.BuildRuntime(cfg, newDisp(), cwd, []string{"OLLIE_SESSION_ID=" + sessID, "OLLIE_UNAME=" + uname, "OLLIE=" + MountPathFor(uname)})
 
 		// sessPtr is set after NewSession; the ReadPlanStep closure captures it.
 		core = agent.NewAgentCore(agent.AgentCoreConfig{