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/index.js

@@ -21,6 +21,14 @@ for (const file of testFiles) {
     const env = {
       GITHUB_OUTPUT: undefined,
       GITHUB_STATE: undefined,
+      HTTP_PROXY: undefined,
+      HTTPS_PROXY: undefined,
+      http_proxy: undefined,
+      https_proxy: undefined,
+      NO_PROXY: undefined,
+      no_proxy: undefined,
+      NODE_OPTIONS: undefined,
+      NODE_USE_ENV_PROXY: undefined,
     };
     const { stderr, stdout } = await execa("node", [`tests/${file}`], { env });
     t.snapshot(stderr, "stderr");