dev: yarn berry from corepack, parked where it cannot clobber pnpm

[landsman]

make install in a Berry project failed with yarn: No such file or directory, and every obvious way of fixing that collides with something already installed.

• brew install yarn is 1.22, which cannot read a __metadata: version 6 lockfile and has no --no-immutable.
• brew install corepack wants to unlink the Brewfile's pnpm — the formulae both own pnpm and pnpx.
• npm i -g corepack fails the same way, EEXIST /opt/homebrew/bin/pnpm.

So make apps installs corepack under its own prefix, where its pnpm and pnpx shims are out of everyone's way, and links only yarn, yarnpkg and corepack into ~/.local/bin. Homebrew's pnpm stays exactly where it was. Nothing here pins a yarn version — corepack reads each project's packageManager field, so a repo asking for yarn@4.7.0 gets 4.7.0.

Two supporting bits: .bashrc now puts ~/.local/bin on PATH, because Ubuntu's ~/.profile only does that if the directory existed at login and on a machine being set up it did not (macOS already had the line in os/macos/.zshrc); and the Brewfile says next to pnpm why yarn has no line of its own there.

Verified by running the recipe against a throwaway $HOME: the three shims appear, no pnpm shim does, and yarn --version in a project pinned to 4.7.0 prints 4.7.0. make qa passes.