chore: ignore .wrangler/, the replacement for the removed .vercel ignores

The Cloudflare migration deleted docs-site/.gitignore's `.vercel` line and the
whole of playground/.gitignore (whose only line was `.vercel`), but never added
the equivalent for wrangler's own local state directory. Both `.wrangler/` dirs
already exist on disk from the cutover deploys; they are empty, so git cannot
see them yet, and the first `wrangler dev` would have dropped untracked state
into two workspaces with nothing ignoring it.

One rule at the root rather than two per-workspace files, since both sites now
have a wrangler.jsonc beside their build output.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01UyzDDiZoywbSHCs66PX15j

Author: ChanMeng666

.gitignore

@@ -25,6 +25,12 @@ dataset/out/
 # local env & agent-session artifacts
 .env
 .env.*
+
+# Wrangler's local state/cache, written beside each site's wrangler.jsonc by
+# `wrangler deploy` / `wrangler dev`. The workspace .gitignore files that used to
+# ignore Vercel's `.vercel` equivalent were removed in the Cloudflare migration;
+# this is the replacement, kept at the root so it covers both sites.
+.wrangler/
 .claude/settings.local.json
 .superpowers/
 .agents/