ATCH_SESSION env var blocks all attach after detach

[DonaldoDes]

Description

After detaching from a session, ATCH_SESSION remains in the shell environment. The self-attach guard in attach_main compares this stale value against the target socket path, blocking all subsequent attach attempts with:

atch: cannot attach to session 'X' from within itself

Reproduction

1. atch -c session1 → detach (Ctrl+)
2. atch list → shows session1
3. atch -a session1 → error "cannot attach from within itself"
4. unset ATCH_SESSION → attach works again

Root cause

The guard relies solely on matching the ATCH_SESSION environment variable against the socket path. It does not verify whether the current process is actually a descendant of the session's shell process.

Suggested fix

Replace the env-var-only check with a PID ancestry walk: read the session's shell PID from a .ppid file written by the master, then walk getppid() upward to confirm actual ancestry before blocking.