feat: add proxy support via child process spawning

When proxy environment variables (https_proxy, HTTPS_PROXY, http_proxy,
HTTP_PROXY) are detected and NODE_USE_ENV_PROXY is not already set to
"1", the action spawns a child process with NODE_USE_ENV_PROXY=1 to
enable Node.js native proxy support.

- Add lib/run-with-proxy.js shared utility for both main.js and post.js
- Update main.js and post.js to use runWithProxy() wrapper
- Add tests for proxy spawning, child error handling, and already-enabled path
- 100% code coverage maintained

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

tests/snapshots/index.js.md

@@ -82,6 +82,36 @@ Generated by [AVA](https://avajs.dev).
     POST /app/installations/123456/access_tokens␊
     {"repositories":["create-github-app-token"]}`
 
+## main-proxy-already-enabled.test.js
+
+> stderr
+
+    ''
+
+> stdout
+
+    ''
+
+## main-proxy-child-error.test.js
+
+> stderr
+
+    ''
+
+> stdout
+
+    ''
+
+## main-proxy-spawns-child.test.js
+
+> stderr
+
+    ''
+
+> stdout
+
+    ''
+
 ## main-repo-skew.test.js
 
 > stderr
@@ -333,6 +363,26 @@ Generated by [AVA](https://avajs.dev).
     POST /app/installations/123456/access_tokens␊
     {"repositories":["create-github-app-token"],"permissions":{"issues":"write","pull_requests":"read"}}`
 
+## post-proxy-already-enabled.test.js
+
+> stderr
+
+    ''
+
+> stdout
+
+    ''
+
+## post-proxy-spawns-child.test.js
+
+> stderr
+
+    ''
+
+> stdout
+
+    ''
+
 ## post-revoke-token-fail-response.test.js
 
 > stderr