fix: push wiki to main branch instead of master

Gitea wiki repos default to "main" branch, but sync_wiki was pushing
to "master", creating a separate branch that never updated the default.
The verification step clones "main" and fails to find the pushed pages.

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Co-Authored-By: Devin <158243242+devin-ai-integration[bot]@users.noreply.github.com>

src/devx/ci/sync_wiki.py

@@ -252,7 +252,7 @@ def commit_and_push(wiki_dir: Path, wiki_url: str, dry_run: bool) -> bool:
 
     # Push
     result = subprocess.run(  # nosec
-        ["git", "push", "--force", wiki_url, "HEAD:master"],
+        ["git", "push", "--force", wiki_url, "HEAD:main"],
         cwd=wiki_dir,
         capture_output=True,
         text=True,