Canonical CI cleanup: TagBot thin-caller + downgrade-caller cleanup (#310)

- TagBot.yml: replace inlined JuliaRegistries/TagBot with canonical
  SciML/.github tagbot.yml@v1 thin caller.
- Downgrade.yml: drop hand-listed stdlib skip and pin julia-version to lts.

Co-authored-by: ChrisRackauckas-Claude <accounts@chrisrackauckas.com>
Co-authored-by: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

Author: 3 people

.github/workflows/Downgrade.yml

@@ -18,7 +18,6 @@ jobs:
     # wall) — red until ForwardDiff 1.3.4 (#811). Auto-greens on upstream release.
     uses: "SciML/.github/.github/workflows/downgrade.yml@v1"
     with:
-      julia-version: "1.10"
+      julia-version: "lts"
       group: "Core"
-      skip: "Pkg,TOML"
     secrets: "inherit"

.github/workflows/TagBot.yml

@@ -1,15 +1,9 @@
-name: TagBot
+name: "TagBot"
 on:
   issue_comment:
-    types:
-      - created
+    types: [created]
   workflow_dispatch:
 jobs:
-  TagBot:
-    if: github.event_name == 'workflow_dispatch' || github.actor == 'JuliaTagBot'
-    runs-on: ubuntu-latest
-    steps:
-      - uses: JuliaRegistries/TagBot@v1
-        with:
-          token: ${{ secrets.GITHUB_TOKEN }}
-          ssh: ${{ secrets.DOCUMENTER_KEY }}
+  tagbot:
+    uses: "SciML/.github/.github/workflows/tagbot.yml@v1"
+    secrets: "inherit"