Document idempotent start (noclobber PID claim) in READMEHEADmaster

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

1 files changed, 4 insertions, 1 deletions

diff --git a/README.md b/README.md
index f62c96d..75b92b1 100644
--- a/README.md
+++ b/README.md
@@ -71,7 +71,10 @@ breaktimer.sh status     # print state, phase, seconds remaining
 refuse if a daemon is already running.)
 
 Auto-start on login by adding `breaktimer.sh start` to your compositor's
-autostart (e.g. Hyprland `exec-once`, Sway `exec`).
+autostart (e.g. Hyprland `exec-once`, Sway `exec`). `start` is idempotent:
+`run_loop` claims the PID file atomically (`noclobber`), so a second `start` —
+or a fast double-login — can't spawn a duplicate daemon; the loser aborts and a
+stale PID file from a previous session is taken over, not duplicated.
 
 ## Configuration