v1.2.0

Release v1.2.0

Fix several bugs in exec::perform():
- initialise ret (was returned uninitialised)
- always nullptr-terminate the environment passed to execvpe
- _exit() in a forked child when exec fails, instead of returning
  into the caller and duplicating its work
- report the child's pid (not the parent's) to the hooks and status
- honour the wait flag so fork + wait=false is fire-and-forget
- decode the wait status into a conventional exit code (128 + signal
  if killed) for perform()/status.code()
- include <sys/types.h> so exec.hpp is self-contained (pid_t)

Tidy the example, document the behavioural options and fix the
assignment snippet in the README, and refresh the GitHub Actions
(checkout@v5, create/upload actions @v2, push/PR build triggers,
drop the unused submodule step).

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
Signed-off-by: Oskari Rauta <oskari.rauta@gmail.com>