watchpr: start the conflict window at the arm

The window ran from the first non-mergeable poll even while the rule was
disarmed, so the poll that armed it confirmed a run it played no part in.
Measure from the later of the run's start and the arm; re-arming stays a
no-op so a base moving under every poll still confirms.

README.md

@@ -94,15 +94,21 @@ watched, and the baseline line — written to stderr, as a JSON record under
 Gitea reports `mergeable: false` both for a real conflict and while it
 recomputes the merge base after a push, so a conflict is only reported once the
 watch has seen a merge computation start (the PR was mergeable, or its head or
-base commit moved) **and** the non-mergeable polls have then run unbroken for
-two minutes. The debounce is a duration, not a poll count, because what it has
-to outlast is Gitea's recompute and `--interval` ranges from seconds to hours.
+base commit moved) **and** the non-mergeable polls have **then** run unbroken for
+two minutes. The window runs from whichever came later, so the poll that starts
+the merge computation never confirms a run of falses that predates it. The
+debounce is a duration, not a poll count, because what it has to outlast is
+Gitea's recompute and `--interval` ranges from seconds to hours.
 
-One case is therefore never reported: a conflict introduced by the push
-immediately before the watch started, on a PR whose head and base never move
-again. Nothing in Gitea's payload separates that from a merge check still in
-flight, so watchpr stays silent about it for as long as it runs — the baseline
-line is how you see it.
+So a conflict introduced by the push immediately before the watch started is not
+reported while the commits stay put: nothing in Gitea's payload separates it from
+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.
 
 ```bash
 # Watch until something meaningful happens (default interval 60s)