gh-140006: Harden fish prompt against shadowed builtins (#150936)

gh-140006: Harden fish prompt against shadowed builtins (#150936)

A user function that shadows the `.`/`source` builtin hijacks the
activate.fish prompt. fish resolves functions ahead of builtins, so the
`echo "exit $status" | .` line that restores the exit status pipes into
the user function instead of sourcing. Dot-style directory navigators
redefine `.`, which made the prompt list the directory on every command
and dropped the exit status handed to the original prompt.

Route every builtin the prompt path uses (`source`, `echo`, `printf`,
`set_color`, `functions`) through `builtin` so no user function can
intercept them. These have all been fish builtins with a stable
interface since fish 2.0.0, the version the script already required
through `functions --copy`, so the minimum supported fish version does
not change.