contributions: issues and pull requests are open

The workflow that closed every pull request on arrival is gone, the README's closed-doors
notice becomes a short Contributing section, and CONTRIBUTING.md says how to report a bug.

Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>

.github/workflows/close-prs.yml

@@ -1,21 +0,0 @@
-# GitHub has no switch to disable pull requests, so this closes them on arrival.
-# Uses pull_request_target (base-repo context, no PR code checked out) so it can
-# comment and close on PRs from forks.
-name: close-prs
-on:
-  pull_request_target:
-    types: [opened, reopened]
-permissions:
-  pull-requests: write
-jobs:
-  close:
-    runs-on: ubuntu-latest
-    steps:
-      - run: |
-          gh pr comment "$PR" --repo "$REPO" --body \
-            "Thanks for the interest! benilla is a solo project and isn't taking pull requests for now — please bring bug reports and ideas to the Discord instead: https://discord.gg/wJSJx467G4"
-          gh pr close "$PR" --repo "$REPO"
-        env:
-          GH_TOKEN: ${{ github.token }}
-          PR: ${{ github.event.pull_request.number }}
-          REPO: ${{ github.repository }}