Add share-ttl: auto-expire shares after configurable timeout

[argusbot78]

Summary

Shared sessions now expire automatically after a configurable timeout (share-ttl, default: 1 hour). Running ds --share on an already-shared session resets the timer. Set share-ttl = 0 to disable.

Behavior

• On ds --share: starts a background watcher that fires ds --unshare after share-ttl seconds; prints ds: share will auto-expire in 3600s (run 'ds --share' to reset)
• On ds --share when already sharing: cancels the old watcher, starts a new one; prints ds: already sharing session '..' (TTL reset to 3600s)
• On ds --unshare / kill / killall: cancels watcher as part of cleanup
• TTL = 0: no watcher spawned, no expiry

Implementation

• New config key share-ttl / env DS_UPTERM_SHARE_TTL (default: 3600)
• _upterm_ttl_pid_file: state file for watcher PID (.upterm.ttl.pid)
• _upterm_start_ttl_watcher: spawns detached background process via setsid (or plain subshell fallback) — sleep TTL && ds --unshare SESSION; saves PID for later cancellation
• _upterm_cancel_ttl_watcher: SIGTERMs the process group to kill both the shell and the sleep child
• _share_start_success helper: consolidates the three success paths in _share_start to avoid repeating watcher-start logic
• _share_stop: calls _upterm_cancel_ttl_watcher before cleanup
• README: new share-ttl row in config table + "Share TTL" section
• examples/share-upterm.conf: share-ttl=3600 added (commented out)

Notes

• ds_bin is resolved via command -v ds before forking, with printf %q escaping for safety
• The watcher runs outside the main shell so it survives terminal detach
• If ds --unshare is called manually before the timer fires, _share_stop cancels the watcher cleanly