Canonical CI cleanup: TagBot thin-caller + downgrade-caller cleanup (#154)

Replace inlined JuliaRegistries/TagBot workflow with the canonical
SciML/.github tagbot.yml@v1 thin caller. Clean up the Downgrade caller:
drop the hand-listed stdlib skip input (now auto-populated centrally)
and pin julia-version to the LTS floor.

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

@@ -19,7 +19,6 @@ jobs:
           - Core
     uses: "SciML/.github/.github/workflows/downgrade.yml@v1"
     with:
-      julia-version: "1.10"
+      julia-version: "lts"
       group: "${{ matrix.group }}"
-      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"