ci: pin mintlify@4.2.284, remove libsecret workaround

Newer unpinned mint versions added openid-client which transitively
requires keytar and libsecret on Linux. Pin to the same version used
locally to avoid the issue entirely.

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

Author: pengying

.github/workflows/lint.yml

@@ -33,14 +33,11 @@ jobs:
           node-version: '24'
           cache: 'npm'
 
-      - name: Install libsecret
-        run: sudo apt-get update && sudo apt-get install -y libsecret-1-dev
-
       - name: Install dependencies
         run: npm ci
 
-      - name: Install mint
-        run: npm i -g mint
+      - name: Install mintlify
+        run: npm i -g mintlify@4.2.284
 
       - name: Lint
         run: make lint