fix: install bun to /usr/local so non-root runner user can access it

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

.github/docker/Dockerfile.ci

@@ -22,10 +22,9 @@ RUN curl -fsSL https://deb.nodesource.com/setup_22.x | bash - \
     && apt-get install -y --no-install-recommends nodejs \
     && rm -rf /var/lib/apt/lists/*
 
-# Bun
+# Bun (install to /usr/local so non-root users can access it)
+ENV BUN_INSTALL="/usr/local"
 RUN curl -fsSL https://bun.sh/install | bash
-ENV BUN_INSTALL="/root/.bun"
-ENV PATH="$BUN_INSTALL/bin:$PATH"
 
 # Claude CLI
 RUN npm i -g @anthropic-ai/claude-code