fix: disable all Docker layer caching in CI

GHA cache was serving stale npm install layers despite cache-bust ARG.
Remove all caching — every build is now fully clean.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Dockerfile

@@ -64,10 +64,6 @@ RUN curl -fsSL https://deb.nodesource.com/setup_lts.x | bash - && \
 # Install Happy Coder globally via npm (stable, rarely changes)
 RUN npm install -g happy-coder
 
-# Cache-bust: tools below fetch "latest" at build time — a changing ARG
-# forces Docker to re-run these layers instead of serving stale cache.
-ARG TOOLS_CACHEBUST=0
-
 # Install Claude Code globally via npm
 RUN npm install -g @anthropic-ai/claude-code