Auto-close PRs and check alphabetical order

- readme_pr_repo_first_commit.py:
  - Close a PR when every added GitHub repo is confirmed younger than
    the 6-month minimum. Unknown/unparseable dates are treated as
    comment-only and do not trigger close.
  - Append an alphabetical-order section to the marker comment when
    the PR adds a list item out of order in any `<details><h2>` or
    `<h3>` sub-section of README.md. Only flags violations that
    involve a line added by the PR.
  - Reconstructs the post-PR README from base + diff so the check
    works on PRs from forks without needing fork-head access.
- readme_pr_alphabetical.py (new): shared helper for the alphabetical
  check.
- workflows/awesome_bot.yaml: removed. awesome_bot was running on
  every push and PR, frequently failing on transient network issues
  and 404s.
- workflows/link_check.yml (new): weekly lychee check on README.md
  (Mon 06:17 UTC + manual dispatch). Tolerant of transient errors,
  posts results back to the Actions run summary.

.github/workflows/link_check.yml

@@ -0,0 +1,40 @@
+name: Link Checker
+
+on:
+  schedule:
+    - cron: '17 6 * * 1'
+  workflow_dispatch:
+
+permissions:
+  contents: read
+  issues: write
+
+concurrency:
+  group: link-check
+  cancel-in-progress: false
+
+jobs:
+  link-check:
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v4
+
+      - name: Lychee link checker
+        uses: lycheeverse/lychee-action@v2
+        env:
+          GITHUB_TOKEN: ${{ secrets.GITHUB_TOKEN }}
+        with:
+          args: >-
+            --verbose
+            --no-progress
+            --accept 100,101,200,202,203,204,205,206,300,301,302,303,304,307,308,403,429
+            --retry 3
+            --retry-wait-time 5
+            --timeout 20
+            --max-redirects 5
+            --exclude-loopback
+            --exclude-mail
+            --exclude 'github\.com/.*/(issues|pulls|actions)'
+            README.md
+          output: github
+          fail: true