Simplify session lifecycle and MCP cleanup

internal/scratchpad/scratchpad.go

@@ -13,10 +13,12 @@ import (
 	"path/filepath"
 	"sort"
 	"strings"
+	"sync"
 )
 
 // Store is the per-project scratchpad directory.
 type Store struct {
+	mu  sync.Mutex
 	dir string
 }
 
@@ -55,6 +57,8 @@ type Entry struct {
 func (s *Store) Dir() string { return s.dir }
 
 func (s *Store) List() ([]Entry, error) {
+	s.mu.Lock()
+	defer s.mu.Unlock()
 	entries, err := os.ReadDir(s.dir)
 	if err != nil {
 		return nil, err
@@ -79,6 +83,8 @@ func (s *Store) List() ([]Entry, error) {
 }
 
 func (s *Store) Read(name string) (content string, revision string, err error) {
+	s.mu.Lock()
+	defer s.mu.Unlock()
 	p, err := s.safePath(name)
 	if err != nil {
 		return "", "", err
@@ -106,6 +112,8 @@ func (e *RevisionMismatchError) Error() string {
 // must match the current revision or the write is rejected with a
 // *RevisionMismatchError (SPEC §14 last-write-wins-with-token).
 func (s *Store) Write(name, content, expectedRevision string) (string, error) {
+	s.mu.Lock()
+	defer s.mu.Unlock()
 	p, err := s.safePath(name)
 	if err != nil {
 		return "", err
@@ -125,6 +133,8 @@ func (s *Store) Write(name, content, expectedRevision string) (string, error) {
 }
 
 func (s *Store) Append(name, content string) error {
+	s.mu.Lock()
+	defer s.mu.Unlock()
 	p, err := s.safePath(name)
 	if err != nil {
 		return err