docs: add security reporting policy (#2561)

* docs(security): add private reporting policy

Document supported versions and route vulnerability reports through GitHub's private advisory workflow.

Add English and Chinese responsible-use notices to the READMEs.

Closes #2544

* ci: skip unrelated pull request builds

Use pull-request-aware path filtering for required Core, GUI, Mobile, and Test workflows so they still publish required check contexts without launching expensive jobs for documentation changes.

Limit the optional OHOS pull request workflow to relevant paths.

README.md

@@ -300,6 +300,12 @@ sudo easytier-core --network-name mysharednode --network-secret mysharednode
 
 EasyTier is released under the [LGPL-3.0](https://github.com/EasyTier/EasyTier/blob/main/LICENSE).
 
+## Responsible Use
+
+Use EasyTier only for lawful purposes and in compliance with applicable laws
+and regulations. You are responsible for ensuring that you are authorized to
+connect to and administer the networks and devices involved.
+
 ## Sponsor
 
 CDN acceleration and security protection for this project are sponsored by Tencent EdgeOne.