Rename CI credentials to org-wide names, demote non-secrets to variables

Four GitHub Apps were reachable under three different key suffixes
(_PRIVATE_KEY, _KEY_, _SECRET) and their ids sat half in secrets, half
in variables. Each app now reads <APP>_ID from variables and
<APP>_PRIVATE_KEY from secrets:

  vars.APP_ID / secrets.APP_PRIVATE_KEY   -> GHAPP_HEADERS_*
  secrets.APP_{ID,KEY}_APPROVE_AND_MERGE  -> GHAPP_MERGEBOT_*
  vars.PUSH_MAIN_APP_ID / secrets.PUSH_MAIN_APP_SECRET -> GHAPP_SYNC_*
  secrets.PB_BOT_APP_{ID,PRIVATE_KEY}     -> GHAPP_PBBOT_*

Values that are not credentials become variables, so a failing run shows
what it talked to instead of ***:

  secrets.POCKETBASE_URL        -> vars.POCKETBASE_URL
  secrets.POCKETBASE_COLLECTION -> vars.POCKETBASE_COLLECTION
  secrets.FRONTEND_URL          -> vars.FRONTEND_URL (also replaces vars.SITE_URL)

The frontend endpoints shared three secrets where two suffice. Cache
revalidation and screenshot import have the same blast radius and merge;
the advisory ingest keeps its own secret because it feeds the update
helper on user systems:

  REVALIDATE_SECRET, SCREENSHOT_IMPORT_SECRET -> FRONTEND_INGEST_SECRET
  BREAKING_CHANGE_INGEST_SECRET               -> FRONTEND_ADVISORY_SECRET

PAT_MICHEL ties infrastructure to one person and existed at both org and
repo level, so the repo copy silently shadowed the org one; it becomes
GH_CROSS_REPO_TOKEN.

.github/workflows/close_issue_in_dev.yaml

@@ -128,7 +128,7 @@ jobs:
         if: steps.find_issue.outputs.issue_numbers != ''
         shell: bash
         env:
-          GH_TOKEN: ${{ secrets.PAT_MICHEL }}
+          GH_TOKEN: ${{ secrets.GH_CROSS_REPO_TOKEN }}
           PR_NUMBER: ${{ github.event.pull_request.number }}
           ISSUE_NUMBERS: ${{ steps.find_issue.outputs.issue_numbers }}
         run: |
@@ -147,8 +147,8 @@ jobs:
       - name: Set is_dev to false in PocketBase
         if: steps.get_slugs.outputs.count != '0'
         env:
-          POCKETBASE_URL: ${{ secrets.POCKETBASE_URL }}
-          POCKETBASE_COLLECTION: ${{ secrets.POCKETBASE_COLLECTION }}
+          POCKETBASE_URL: ${{ vars.POCKETBASE_URL }}
+          POCKETBASE_COLLECTION: ${{ vars.POCKETBASE_COLLECTION }}
           POCKETBASE_ADMIN_EMAIL: ${{ secrets.POCKETBASE_ADMIN_EMAIL }}
           POCKETBASE_ADMIN_PASSWORD: ${{ secrets.POCKETBASE_ADMIN_PASSWORD }}
           PR_URL: ${{ github.server_url }}/${{ github.repository }}/pull/${{ github.event.pull_request.number }}