fix: terminal clear, detach, log replay, self-attach, and umask bugs

[s85io]

Summary

Collection of bug fixes for attach/detach lifecycle and master socket creation:

• Replace RIS with explicit clear sequences — avoid leaking terminal state (bold, colors, charset) when clearing screen on attach/detach. Uses \033[999H\r\n (move-to-bottom) instead of full terminal reset.
• Send MSG_DETACH on detach keypress — the client now notifies the master process before exiting, so the master can properly clean up the client entry.
• Limit log replay to last SCROLLBACK_SIZE bytes — large session logs were read from the start, causing slow replays. Now seeks to the tail when the log exceeds SCROLLBACK_SIZE.
• Move connect_socket before self-attach check — a stale SESSION_ENVVAR from a dead session would block new attaches. Now we connect first and only then check for loops. Also closes the socket on self-attach detection.
• Separate ENOENT from replay path — "session does not exist" is now shown directly instead of falling through to the log replay code path, giving clearer error messages.
• Restore umask on all error paths in create_socket — umask(omask) was only called on the success path; failures in bind/listen/setnonblocking/chmod left the process umask changed to 077.

Test plan

• Build and install (make clean && make && sudo cp atch /usr/bin/atch)
• Verify session create, list, list -a
• Attach to session, press detach key, verify clean disconnect
• Kill a session, verify list -a shows it as exited, re-attach replays log tail
• Verify no self-attach from within a session
• Run long-lived session with large output, verify replay is fast

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