fix: require NODE_USE_ENV_PROXY for proxy support (#342)

This PR switches proxy support to Node's native env-proxy handling and
makes the required configuration explicit.

## What changed

- fail fast in both `main` and `post` when proxy configuration is
present without `NODE_USE_ENV_PROXY=1`
- document the supported proxy configuration in `README.md`
- add regression tests for the proxy guard in both entrypoints
- keep the existing successful end-to-end coverage and add a smaller
proxy-specific workflow check that enables native proxy support, points
`https_proxy` at an unreachable proxy, and asserts the action fails
- update the test workflow so the same checks also run on pushes to
`beta`

## Proxy configuration

When using `HTTP_PROXY` or `HTTPS_PROXY`, set `NODE_USE_ENV_PROXY=1` on
the action step. If you need bypass rules, set `NO_PROXY` alongside
them.

---------

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

tests/main-proxy-requires-native-support.test.js

@@ -0,0 +1,14 @@
+process.env.GITHUB_REPOSITORY = "actions/create-github-app-token";
+process.env.GITHUB_REPOSITORY_OWNER = "actions";
+process.env.HTTPS_PROXY = "http://127.0.0.1:3128";
+
+const originalConsoleError = console.error;
+console.error = (...args) => {
+  originalConsoleError(
+    ...args.map((arg) => (arg instanceof Error ? arg.message : arg)),
+  );
+};
+
+await import("../main.js");
+await new Promise((resolve) => setImmediate(resolve));
+process.exitCode = 0;