ci: add standard centralized workflows (#152)

Adds the standard SciML centralized caller workflows that were missing:
- RunicSuggestions.yml
- DependabotAutoMerge.yml
- DocPreviewCleanup.yml

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/DependabotAutoMerge.yml

@@ -0,0 +1,9 @@
+name: "Dependabot Auto-merge"
+on: pull_request
+permissions:
+  contents: write
+  pull-requests: write
+jobs:
+  automerge:
+    uses: "SciML/.github/.github/workflows/dependabot-automerge.yml@v1"
+    secrets: "inherit"

.github/workflows/DocPreviewCleanup.yml

@@ -0,0 +1,10 @@
+name: "Doc Preview Cleanup"
+on:
+  pull_request:
+    types: [closed]
+permissions:
+  contents: write
+jobs:
+  cleanup:
+    uses: "SciML/.github/.github/workflows/docs-preview-cleanup.yml@v1"
+    secrets: "inherit"

.github/workflows/RunicSuggestions.yml

@@ -0,0 +1,9 @@
+name: "Runic Suggestions"
+on: pull_request
+permissions:
+  contents: read
+  pull-requests: write
+jobs:
+  runic-suggestions:
+    uses: "SciML/.github/.github/workflows/runic-suggestions-on-pr.yml@v1"
+    secrets: "inherit"