feat: detect stale config and prompt merge-or-overwrite on save

Records the file mtime at load time; before write_config() fires, re-checks
it. If it changed (e.g. `claude mcp add`, a second BCC window, or Claude
itself writing ~/.claude.json), StaleDialog prompts with the changed top-level
key names and a masked server-section diff. "Merge & save" applies the user's
in-memory server edits on top of the current on-disk file (preserving external
non-server changes); "Overwrite anyway" proceeds as before.

- bcc_core: config_mtime(), external_change_summary(), _server_sections()
  helper extracted from backup_diff for reuse
- bcc.py: StaleDialog, MainWindow._loaded_mtime tracked through load/save
- tests: 7 new tests (config_mtime, external_change_summary variants, AC merge test)

Closes #4

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

bcc.py

@@ -926,6 +926,80 @@ class RestoreDialog(QDialog):
         return item.data(Qt.ItemDataRole.UserRole)
 
 
+# --------------------------------------------------------------------------- #
+# Stale-file conflict dialog
+# Shown when the config file was modified on disk between load and save.
+# --------------------------------------------------------------------------- #
+class StaleDialog(QDialog):
+    MERGE = 0
+    OVERWRITE = 1
+
+    def __init__(
+        self,
+        parent,
+        profile_path: str,
+        changed_keys: list[str],
+        server_diff: str,
+    ):
+        super().__init__(parent)
+        self.setWindowTitle("Config changed on disk")
+        self.resize(680, 420)
+        self._choice = self.MERGE
+
+        v = QVBoxLayout(self)
+        v.setSpacing(8)
+
+        intro = QLabel(f"<b>{profile_path}</b> was modified on disk since you loaded it.")
+        intro.setWordWrap(True)
+        v.addWidget(intro)
+
+        if changed_keys:
+            kl = QLabel("Changed keys: " + ", ".join(f"<code>{k}</code>" for k in changed_keys))
+            kl.setObjectName("muted")
+            kl.setWordWrap(True)
+            v.addWidget(kl)
+
+        if server_diff:
+            v.addWidget(QLabel("Server section changes (on disk vs loaded):"))
+            dv = QPlainTextEdit()
+            dv.setObjectName("diag")
+            dv.setReadOnly(True)
+            dv.setPlainText(server_diff)
+            v.addWidget(dv, 1)
+        else:
+            note = QLabel("(Server sections unchanged — the external edit is in other keys.)")
+            note.setObjectName("muted")
+            v.addWidget(note)
+
+        desc = QLabel(
+            "<b>Merge &amp; save</b> applies your server edits on top of the current file "
+            "on disk — both sets of changes are kept.  "
+            "<b>Overwrite</b> saves your in-memory state, discarding external changes."
+        )
+        desc.setObjectName("muted")
+        desc.setWordWrap(True)
+        v.addWidget(desc)
+
+        btns = QDialogButtonBox()
+        merge_btn = btns.addButton("Merge && save", QDialogButtonBox.ButtonRole.AcceptRole)
+        merge_btn.setObjectName("primary")
+        overwrite_btn = btns.addButton(
+            "Overwrite anyway", QDialogButtonBox.ButtonRole.DestructiveRole
+        )
+        btns.addButton(QDialogButtonBox.StandardButton.Cancel)
+        merge_btn.clicked.connect(lambda: self._choose(self.MERGE))
+        overwrite_btn.clicked.connect(lambda: self._choose(self.OVERWRITE))
+        btns.rejected.connect(self.reject)
+        v.addWidget(btns)
+
+    def _choose(self, choice: int) -> None:
+        self._choice = choice
+        self.accept()
+
+    def choice(self) -> int:
+        return self._choice
+
+
 # --------------------------------------------------------------------------- #
 # Paste-JSON dialog
 # --------------------------------------------------------------------------- #
@@ -1016,6 +1090,7 @@ class MainWindow(QMainWindow):
         self.full_config: dict = {}
         self.servers: list[core.ServerEntry] = []
         self.current_profile: core.Profile | None = None
+        self._loaded_mtime: float | None = None
         self.dirty = False
         self._suppress_table = False
         self._suppress_sel = False
@@ -1291,6 +1366,7 @@ class MainWindow(QMainWindow):
                 return
             self.full_config = cfg
             repaired = True
+        self._loaded_mtime = core.config_mtime(profile.path)
         self.current_profile = profile
         self.servers = core.extract_servers(self.full_config)
         self.dirty = False
@@ -1627,12 +1703,52 @@ class MainWindow(QMainWindow):
         if not self._validate():
             QMessageBox.warning(self, "Can't save yet", "Fix the highlighted problem first.")
             return
+
+        # Stale-file check: if the file changed on disk since we loaded it, prompt.
+        disk_mtime = core.config_mtime(self.current_profile.path)
+        if (
+            disk_mtime is not None
+            and self._loaded_mtime is not None
+            and disk_mtime != self._loaded_mtime
+        ):
+            changed_keys, server_diff = core.external_change_summary(
+                self.full_config, self.current_profile.path
+            )
+            dlg = StaleDialog(self, str(self.current_profile.path), changed_keys, server_diff)
+            if not dlg.exec():
+                return  # user cancelled
+            if dlg.choice() == StaleDialog.MERGE:
+                try:
+                    fresh = core.load_config(self.current_profile.path)
+                except Exception as e:
+                    QMessageBox.critical(self, "Reload failed", str(e))
+                    return
+                core.apply_servers(fresh, self.servers)
+                try:
+                    backup = core.write_config(self.current_profile.path, fresh)
+                except Exception as e:
+                    QMessageBox.critical(self, "Save failed", str(e))
+                    return
+                self.full_config = fresh
+                self._loaded_mtime = core.config_mtime(self.current_profile.path)
+                self.current_profile.config_exists = True
+                self.dirty = False
+                self.save_btn.setEnabled(False)
+                bnote = f"  ·  backup: {backup.name}" if backup else "  ·  (new file)"
+                self.status.setText(
+                    f"Merged & saved {self.current_profile.path}{bnote}"
+                    f"  ·  Restart {self.current_profile.label} to apply."
+                )
+                return
+            # else OVERWRITE: fall through to normal write
+
         core.apply_servers(self.full_config, self.servers)
         try:
             backup = core.write_config(self.current_profile.path, self.full_config)
         except Exception as e:
             QMessageBox.critical(self, "Save failed", str(e))
             return
+        self._loaded_mtime = core.config_mtime(self.current_profile.path)
         self.current_profile.config_exists = True
         self.dirty = False
         self.save_btn.setEnabled(False)