CI: skip GitHub-specific workflows on Gitea Actions; add workflow_dispatch

Gitea reads .github/workflows/ for compatibility, so without guards it
tries to run the release and wiki-sync workflows on Gitea too - which
fail because they use softprops/action-gh-release@v2 (GitHub-only) and
push to GitHub-hosted wiki URLs.

Add `if: github.server_url == 'https://github.com'` to the
release-builder and wiki-sync jobs so Gitea no-ops them. ci.yml
(plain dotnet build + test) still runs on both, and now also accepts
workflow_dispatch so it can be triggered by hand to verify a self-
hosted runner is picking up jobs.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

.github/workflows/wiki-sync.yml

@@ -11,6 +11,10 @@ on:
 
 jobs:
   sync:
+    # Gitea reads .github/workflows/ for compatibility, but this workflow
+    # pushes to a GitHub-hosted wiki. Skip on non-GitHub runners; the Gitea
+    # wiki is synced separately via scripts/sync-wiki.ps1.
+    if: github.server_url == 'https://github.com'
     runs-on: windows-latest
     permissions:
       contents: write