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/snapshots/index.js.md

@@ -82,6 +82,16 @@ Generated by [AVA](https://avajs.dev).
     POST /app/installations/123456/access_tokens␊
     {"repositories":["create-github-app-token"]}`
 
+## main-proxy-requires-native-support.test.js
+
+> stderr
+
+    'A proxy environment variable is set, but Node.js native proxy support is not enabled. Set NODE_USE_ENV_PROXY=1 for this action step.'
+
+> stdout
+
+    '::error::A proxy environment variable is set, but Node.js native proxy support is not enabled. Set NODE_USE_ENV_PROXY=1 for this action step.'
+
 ## main-repo-skew.test.js
 
 > stderr
@@ -333,6 +343,16 @@ 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-requires-native-support.test.js
+
+> stderr
+
+    'A proxy environment variable is set, but Node.js native proxy support is not enabled. Set NODE_USE_ENV_PROXY=1 for this action step.'
+
+> stdout
+
+    '::error::A proxy environment variable is set, but Node.js native proxy support is not enabled. Set NODE_USE_ENV_PROXY=1 for this action step.'
+
 ## post-revoke-token-fail-response.test.js
 
 > stderr