Fix opencode not found: symlink installer binary to /usr/local/bin

The opencode curl installer puts the binary in /root/.local/bin which
isn't on PATH for the nextjs user. Add a symlink to /usr/local/bin
after install. Also ensure /usr/local/bin is always in the PATH
passed to spawned agent processes.

apps/harness/Dockerfile

@@ -42,6 +42,8 @@ RUN apk add --no-cache git github-cli curl ca-certificates
 RUN npm install -g @anthropic-ai/claude-code @openai/codex
 RUN curl -fsSL https://opencode.ai/install | sh || \
     echo "WARN: opencode install failed, skipping"
+# Ensure opencode is on PATH (installer puts it in ~/.local/bin)
+RUN if [ -f /root/.local/bin/opencode ]; then ln -s /root/.local/bin/opencode /usr/local/bin/opencode; fi
 
 # MCP servers: Gitea (Go binary from builder stage)
 COPY --from=gitea-mcp-builder /usr/local/bin/gitea-mcp /usr/local/bin/gitea-mcp