feat(core): add plan file persistence to disk

Save approved plans to ~/.qwen/plans/{sessionId}.md so they can be
referenced later in the session or reviewed outside the CLI.

Changes:
- Storage: add getPlansDir() and getPlanFilePath()
- Config: add savePlan(), loadPlan(), getPlanFilePath() methods
- ExitPlanModeTool: persist plan to disk when user approves
- Tests: 4 new Config tests, 2 new ExitPlanModeTool assertions

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

packages/core/src/config/config.ts

@@ -6,6 +6,7 @@
 
 // Node built-ins
 import type { EventEmitter } from 'node:events';
+import * as fs from 'node:fs';
 import * as path from 'node:path';
 import process from 'node:process';
 
@@ -1665,6 +1666,35 @@ export class Config {
     this.approvalMode = mode;
   }
 
+  /**
+   * Returns the file path for this session's plan file.
+   */
+  getPlanFilePath(): string {
+    return Storage.getPlanFilePath(this.sessionId);
+  }
+
+  /**
+   * Saves a plan to disk for the current session.
+   */
+  savePlan(plan: string): void {
+    const filePath = this.getPlanFilePath();
+    const dir = path.dirname(filePath);
+    fs.mkdirSync(dir, { recursive: true });
+    fs.writeFileSync(filePath, plan, 'utf-8');
+  }
+
+  /**
+   * Loads the plan for the current session, or returns undefined if none exists.
+   */
+  loadPlan(): string | undefined {
+    const filePath = this.getPlanFilePath();
+    try {
+      return fs.readFileSync(filePath, 'utf-8');
+    } catch {
+      return undefined;
+    }
+  }
+
   getInputFormat(): 'text' | 'stream-json' {
     return this.inputFormat;
   }