watchpr: detect merge/close in poll loop (was hanging after baseline)

watchpr only reported meaningful changes as transitions from the poll
baseline. A PR already merged or closed when the watch started was
captured as the baseline and never produced a transition, so the loop
polled the dead PR forever (process alive, never exiting) -- the
single-PR --interval case observed in production.

Add a terminal-state check applied to the baseline snapshot: a PR that
is already merged or closed the moment watchpr starts is reported and
exits immediately, since it can never change again. Extract the
baseline+poll loop into agent.Watch behind a StateFetcher interface so
the loop, its open->merged/close detection, and its poll-error
resilience are unit-testable with a fake client.

cmd/watchpr/main.go

@@ -117,36 +117,23 @@ func runOnce(c *agent.GiteaClient, refs []agent.PRRef, jsonMode bool) error {
 func runWatch(c *agent.GiteaClient, refs []agent.PRRef, interval time.Duration, jsonMode bool) error {
 	login := agent.AgentLogin()
 
-	prev := make(map[string]agent.PRState, len(refs))
-	for _, ref := range refs {
-		st, err := agent.FetchState(c, ref, login)
-		if err != nil {
-			return err
-		}
-		prev[ref.String()] = st
-	}
-	if !jsonMode {
-		fmt.Fprintf(os.Stderr, "watching %d PR(s) every %s; baseline established\n", len(refs), interval)
-	}
-
 	ticker := time.NewTicker(interval)
 	defer ticker.Stop()
-	for range ticker.C {
-		for _, ref := range refs {
-			key := ref.String()
-			cur, err := agent.FetchState(c, ref, login)
-			if err != nil {
-				fmt.Fprintf(os.Stderr, "warning: polling %s: %v\n", key, err)
-				continue
-			}
-			changed, reason := agent.MeaningfulChange(prev[key], cur)
-			if changed {
-				report(key, reason, cur, jsonMode)
-				return nil
-			}
-			prev[key] = cur
+
+	onBaseline := func() {
+		if !jsonMode {
+			fmt.Fprintf(os.Stderr, "watching %d PR(s) every %s; baseline established\n", len(refs), interval)
 		}
 	}
+	onError := func(ref agent.PRRef, err error) {
+		fmt.Fprintf(os.Stderr, "warning: polling %s: %v\n", ref.String(), err)
+	}
+
+	res, err := agent.Watch(c, refs, login, ticker.C, onBaseline, onError)
+	if err != nil {
+		return err
+	}
+	report(res.Ref.String(), res.Reason, res.State, jsonMode)
 	return nil
 }