Write out the PID

If magic-trace wrote out the PID of the process it was attached to when it attached, users could copy+paste that the next time they invoke magic-trace. I already do that with the symbol after using `-trigger ?`, and it'd be useful to do it for PID as well.

Start by tracing magic-trace's attach flow and the existing output produced for `-trigger ?`. Add equivalent output for the attached process's PID, then verify that users can copy it for a later invocation.

Written by the indexing model from the issue text.