refactor: break process_title->cli by moving initial_title to cli::proctitle

initial_title/set_initial_title map the parsed clap Command/Args enum to a
process title string, so they intrinsically depend on the cli arg types. Move
them up into a new cli::proctitle submodule. The low-level title primitives
(compact_process_title, session_name, set_title) stay in the process_title
core module and are now pub(crate).

Removes the process_title->cli edge (3 refs: Args/Command import +
client_selfdev_requested x2), making process_title a pure core module.