watchpr: describe arming as a one-way latch, not a window restart

The docs claimed every later head/base move restarts the conflict window.
arm() returns early once armed, so only the first move sets armedAt and
every move after it is a no-op. State the real trade-off instead: repeated
moves do not extend the debounce, so a conflict can be confirmed while the
newest recompute is younger than the window.

Also narrow the --json stderr claim to the notices watchpr writes itself
(cobra's terminal Error: line is plain text), rename the test to what it
covers, and stop the unknown-mergeability baseline implying a false answer
arms the rule.

README.md

@@ -105,10 +105,18 @@ reported while the commits stay put: nothing in Gitea's payload separates it fro
 a merge check still in flight, and the baseline line is the only notice of it.
 That is a narrower gap than it looks, because `base.sha` is the base branch's
 tip — it moves for every open PR whenever anything merges to the base branch, so
-the commits rarely stay put for long. A move like that arms the rule, and the
-move itself is silent: the window restarts from it, so what is eventually
-reported is a conflict still standing two minutes after that fresh merge
-computation began, never the run of falses that preceded it.
+the commits rarely stay put for long. The first such move arms the rule, and is
+itself silent: the window then runs from that arming poll rather than from the
+falses that predate it, so what is eventually reported is a conflict that
+outlasted the merge computation the move started.
+
+Arming is a one-way latch. Moves after that one — and every move seen by a watch
+that was already armed at the baseline — neither re-arm nor restart the window,
+because a restart on every `base.sha` move could never complete on a busy base
+branch. That is the cost side of the same trade: on a busy base branch a
+conflict can be confirmed while the newest merge recompute is less than two
+minutes old. The window guarantees that the run of falses outlasted *a* merge
+computation this watch saw start, not that it outlasted the most recent one.
 
 ```bash
 # Watch until something meaningful happens (default interval 60s)