Remove stale npx workflow caches

[cbusillo]

Summary

• remove obsolete npm/npx cache steps from issue-code and upstream-merge workflows
• keep Node setup because both workflows still run the local OpenAI proxy with node
• continue using the repo-built CODE_BIN path for agent execution

Validation

• git diff --check
• rg -n "Cache npm \(npx\)|npx -y @just-every/code@latest|@just-every/code@latest" .github/workflows/issue-code.yml .github/workflows/upstream-merge.yml .github/workflows/issue-comment.yml (no matches)
• ./build-fast.sh

Refs #130