fix(deploy): stop the staging web image from racing two copies of skills/

Next's output file tracing pulls skills/ into .next/standalone with symlinks
resolved into real directories. The final stage copies the standalone bundle
into /app first, so the later explicit `COPY skills /app/skills` tries to put
the tracked symlink skills/jyotish-vedic-astrology/assets -> ../../assets on
top of a real directory. Whether that is fatal depends on the BuildKit
snapshotter: GitHub's runners tolerate it, Docker 29.1.3 with
containerd-overlayfs refuses with "cannot replace to directory ... with file".

Dropping the traced copy after the build lets the explicit COPY own that tree
deterministically. /app/assets still comes from the standalone bundle, which
outputFileTracingIncludes guarantees, so the symlink resolves.

Verified by building the image and inspecting it: skills/ complete at 1208
files, assets still a symlink that resolves, container boots on Next.js
16.3.1 and serves / with HTTP 200. Reproduced the original failure on the
pre-upgrade commit too, so it predates the Next 16.3.1 upgrade.

Co-authored-by: Cursor <cursoragent@cursor.com>

deploy/railway-web.Dockerfile

@@ -25,6 +25,10 @@ COPY skills /app/skills
 
 RUN npm run build
 
+# File tracing pulls skills/ into the standalone bundle with symlinks resolved into
+# real directories, which then collides with the explicit COPY in the final stage.
+RUN rm -rf /app/frontend/.next/standalone/skills
+
 FROM swr.cn-north-4.myhuaweicloud.com/ddn-k8s/docker.io/library/node:22-alpine
 
 WORKDIR /app/frontend