Make the wiki publisher a scoped workflow with an opt-in gate

Embed the publish script in .gitea/scoped_workflows/publish-wiki.yml and
remove the repository workflow and tools/publish_wiki.py. A repository
publishes only when wiki/.publish exists; the word prune in it is the only
thing that lets the workflow delete pages. The organisation's
TOKEN_FOR_GITEA_WIKI is used with no fallback. This repository opts in with
prune. Tests run the embedded script against a fake Gitea.

Task: MIL-006#2
Task: MIL-006#3

.gitea/workflows/publish-wiki.yml

@@ -1,32 +0,0 @@
-name: Publish wiki
-
-on:
-  push:
-    branches: [ main ]
-    paths:
-      - "wiki/**"
-  workflow_dispatch:
-
-concurrency:
-  group: publish-wiki
-  cancel-in-progress: false
-
-jobs:
-  publish-wiki:
-    permissions:
-      contents: read
-    runs-on: ubuntu-latest
-    steps:
-      - name: Check out the repository
-        uses: actions/checkout@v4
-
-      - name: Check that the English pages are current
-        run: python3 tools/build_wiki.py --check
-
-      - name: Publish the pages to the wiki
-        env:
-          GITEA_API_URL: ${{ gitea.api_url }}
-          SOURCE_REPOSITORY: ${{ gitea.repository }}
-          # TOKEN_FOR_GITEA_WIKI wins when it exists; otherwise TOKEN_FOR_GITEA is reused.
-          WIKI_CREDENTIALS: ${{ secrets.TOKEN_FOR_GITEA_WIKI || secrets.TOKEN_FOR_GITEA }}
-        run: python3 tools/publish_wiki.py