feat(core): add prePlanMode tracking to restore previous approval mode

When entering plan mode, Config now saves the previous approval mode
(e.g. AUTO_EDIT, YOLO) so it can be restored when exiting. Previously,
/plan execute and ExitPlanModeTool both hardcoded a return to DEFAULT,
losing the user's prior mode.

Changes:
- Config: add prePlanMode field, getPrePlanMode(), auto-track in
  setApprovalMode()
- planCommand: /plan execute restores prePlanMode instead of DEFAULT
- ExitPlanModeTool: ProceedOnce restores prePlanMode instead of DEFAULT
- Tests: 4 new Config tests, 1 new planCommand test, 1 new
  ExitPlanModeTool test

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

packages/core/src/config/config.ts

@@ -529,6 +529,7 @@ export class Config {
   private sdkMode: boolean;
   private geminiMdFileCount: number;
   private approvalMode: ApprovalMode;
+  private prePlanMode: ApprovalMode | undefined;
   private readonly accessibility: AccessibilitySettings;
   private readonly telemetrySettings: TelemetrySettings;
   private readonly gitCoAuthor: GitCoAuthorSettings;
@@ -1634,6 +1635,14 @@ export class Config {
     return this.approvalMode;
   }
 
+  /**
+   * Returns the approval mode that was active before entering plan mode.
+   * Falls back to DEFAULT if no pre-plan mode was recorded.
+   */
+  getPrePlanMode(): ApprovalMode {
+    return this.prePlanMode ?? ApprovalMode.DEFAULT;
+  }
+
   setApprovalMode(mode: ApprovalMode): void {
     if (
       !this.isTrustedFolder() &&
@@ -1644,6 +1653,15 @@ export class Config {
         'Cannot enable privileged approval modes in an untrusted folder.',
       );
     }
+    // Track the mode before entering plan mode so it can be restored later
+    if (mode === ApprovalMode.PLAN && this.approvalMode !== ApprovalMode.PLAN) {
+      this.prePlanMode = this.approvalMode;
+    } else if (
+      mode !== ApprovalMode.PLAN &&
+      this.approvalMode === ApprovalMode.PLAN
+    ) {
+      this.prePlanMode = undefined;
+    }
     this.approvalMode = mode;
   }