Replace two scratch paths in design docs, drop their exclusions, and give the Runner job more headroom #164

From #163's review: (1) docs/design/executor-ownership.md and docs/design/manual-admission-store-prepare-shape.md carry session scratch paths (a Go cache directory and a wire capture file); replace them with <scratch> and drop the two whole-file exclusions from the local-paths step. (2) The Runner (ubuntu-latest) job ran in 10m36s against its 15-minute limit, with the unit tests alone near 10 minutes; raise the limit to 25 minutes.