fix(ci): rebuild from clean in size-report so build-config changes are measured
The size report builds head then base without cleaning between them. For PRs that change only build config (Makefile, patches/, flags) and not src/*.c, make sees clayterm.wasm as newer than the unchanged sources and skips the base rebuild, so base re-measures head's artifact, delta is 0, and no size comment is posted. Run make clean && make for both builds so each is from scratch.