feat(#93): filesystem permission pre-flight for credential configs (0600/0700)

ssh-mcp refuses to start if its config is group/world-readable (mode & 0o077 →
throws, requiring dir 0700 / file 0600). A GUI user has no idea what chmod 600
means — they just get a dead server. This checks it and offers a one-click fix.

Core (pure, POSIX-only, injectable stat/chmod so tests never touch a real file):
- permission_status(path): the ssh-mcp rule — any group/other bit set (mode & 0o077)
  is not-ok; file must be 0600, its dir 0700. Returns None on Windows (modes don't
  apply) or when the file is absent (nothing to pre-flight). Plain-language problems
  naming the offending octal mode.
- fix_permissions(path): chmod file → 0600, dir → 0700. No-op on Windows; reports
  an OSError instead of raising.
- sidecar_permission_warnings() / sidecar_permission_fix_target(): tie the check to
  #91's sidecar path resolution so it knows WHICH file to inspect. Mirror the
  sidecar-warnings shape.

GUI: a warning label + "Fix permissions" button in the stdio editor (mirrors the
removed-flag surface), shown only when the sidecar exists and is too open. Hidden
on Windows and for non-sidecar servers. Smoke-tested headlessly.

pytest green (529 passed), ruff + format clean. Closes #93. Part of epic #94.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

bcc.py

@@ -908,6 +908,21 @@ class ServerEditor(QFrame):
         self.sidecar_warn.setWordWrap(True)
         self.sidecar_warn.hide()
         v.addWidget(self.sidecar_warn)
+        # Shown when the sidecar config is group/other-accessible (#93): ssh-mcp
+        # refuses to start unless it's 0600 / its dir 0700. One-click chmod fix.
+        # POSIX only — hidden on Windows where modes don't apply.
+        self.perm_warn = QLabel("")
+        self.perm_warn.setStyleSheet(f"color: {WARN};")
+        self.perm_warn.setWordWrap(True)
+        self.perm_warn.hide()
+        self.perm_fix_btn = QPushButton("Fix permissions")
+        self.perm_fix_btn.setToolTip("chmod the config file to 0600 and its directory to 0700")
+        self.perm_fix_btn.clicked.connect(self._fix_permissions)
+        self.perm_fix_btn.hide()
+        perm_row = QHBoxLayout()
+        perm_row.addWidget(self.perm_warn, 1)
+        perm_row.addWidget(self.perm_fix_btn)
+        v.addLayout(perm_row)
         v.addWidget(self._lbl("Environment variables"))
         self.env = KeyValueTable(
             "Variable", "Value", on_change=self._emit, before_change=self._before_change
@@ -1010,6 +1025,8 @@ class ServerEditor(QFrame):
             self.removed_flag_warn.hide()
             self.removed_flag_fix_btn.hide()
             self.sidecar_warn.hide()
+            self.perm_warn.hide()
+            self.perm_fix_btn.hide()
             self._loading = False
             return
         self.setEnabled(True)
@@ -1100,6 +1117,8 @@ class ServerEditor(QFrame):
             self.removed_flag_warn.hide()
             self.removed_flag_fix_btn.hide()
             self.sidecar_warn.hide()
+            self.perm_warn.hide()
+            self.perm_fix_btn.hide()
             return
         _, notes = core.split_suspicious_args(self._current_arg_lines())
         if notes:
@@ -1140,6 +1159,15 @@ class ServerEditor(QFrame):
             self.sidecar_warn.show()
         else:
             self.sidecar_warn.hide()
+        # Sidecar filesystem permissions (#93). Real platform/fs; no-op on Windows.
+        perm_warnings = core.sidecar_permission_warnings(stdio)
+        if perm_warnings:
+            self.perm_warn.setText("⚠  " + "\n".join(perm_warnings))
+            self.perm_warn.show()
+            self.perm_fix_btn.show()
+        else:
+            self.perm_warn.hide()
+            self.perm_fix_btn.hide()
 
     def _fix_args(self):
         if self._before_change:
@@ -1160,6 +1188,23 @@ class ServerEditor(QFrame):
         self.env.load(migrated.get("env", {}))
         self.args.setPlainText("\n".join(migrated.get("args", [])))
 
+    def _fix_permissions(self):
+        """chmod the sidecar config to 0600 / its dir to 0700 (#93)."""
+        stdio = {
+            "command": self.command.text().strip(),
+            "args": self._current_arg_lines(),
+            "env": self.env.dump(),
+        }
+        target = core.sidecar_permission_fix_target(stdio)
+        if target is None:
+            return
+        changed, note = core.fix_permissions(target)
+        if not changed:
+            # Surface the failure in-place rather than silently doing nothing.
+            self.perm_warn.setText("⚠  " + (note or "could not change permissions"))
+            return
+        self._check_args()  # re-check; the warning clears when perms are now tight
+
     # --- dependency ------------------------------------------------------ #
     def refresh_dependency(self, auto_open=False):
         if not self.isEnabled():