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.

internal/agent/watch.go

@@ -1,5 +1,7 @@
 package agent
 
+import "time"
+
 // PRState is a point-in-time snapshot of the PR attributes watchpr tracks.
 type PRState struct {
 	Ref              PRRef  `json:"ref"`
@@ -41,6 +43,78 @@ func FetchState(c *GiteaClient, ref PRRef, agentLogin string) (PRState, error) {
 	}, nil
 }
 
+// StateFetcher fetches the current PRState for a ref. *GiteaClient satisfies it
+// via its FetchState method; tests inject fakes.
+type StateFetcher interface {
+	FetchState(ref PRRef, agentLogin string) (PRState, error)
+}
+
+// FetchState makes *GiteaClient a StateFetcher.
+func (c *GiteaClient) FetchState(ref PRRef, agentLogin string) (PRState, error) {
+	return FetchState(c, ref, agentLogin)
+}
+
+// WatchResult is the change that ended a watch.
+type WatchResult struct {
+	Ref    PRRef
+	Reason string
+	State  PRState
+}
+
+// terminalState reports whether a PR has reached a final state from which no
+// further meaningful change is possible, with a human-readable reason. Unlike a
+// transition (see MeaningfulChange) this holds for a single snapshot, so it also
+// catches a PR that is already merged/closed the moment watchpr starts.
+func terminalState(st PRState) (bool, string) {
+	if st.Merged {
+		return true, "PR merged"
+	}
+	if st.State == "closed" {
+		return true, "PR closed without merging"
+	}
+	return false, ""
+}
+
+// Watch establishes a baseline for each ref, then polls on every tick until a
+// tracked PR changes meaningfully, returning the first such change. A PR that is
+// already terminal (merged/closed) at baseline is reported immediately rather
+// than polled forever. Poll errors are handed to onError and never stop the
+// loop; only a baseline fetch error aborts. onBaseline, if set, fires once after
+// all baselines are captured and before the first tick.
+func Watch(f StateFetcher, refs []PRRef, agentLogin string, ticks <-chan time.Time, onBaseline func(), onError func(PRRef, error)) (WatchResult, error) {
+	prev := make(map[string]PRState, len(refs))
+	for _, ref := range refs {
+		st, err := f.FetchState(ref, agentLogin)
+		if err != nil {
+			return WatchResult{}, err
+		}
+		if terminal, reason := terminalState(st); terminal {
+			return WatchResult{Ref: ref, Reason: reason, State: st}, nil
+		}
+		prev[ref.String()] = st
+	}
+	if onBaseline != nil {
+		onBaseline()
+	}
+	for range ticks {
+		for _, ref := range refs {
+			key := ref.String()
+			cur, err := f.FetchState(ref, agentLogin)
+			if err != nil {
+				if onError != nil {
+					onError(ref, err)
+				}
+				continue
+			}
+			if changed, reason := MeaningfulChange(prev[key], cur); changed {
+				return WatchResult{Ref: ref, Reason: reason, State: cur}, nil
+			}
+			prev[key] = cur
+		}
+	}
+	return WatchResult{}, nil
+}
+
 // countNonAgentComments counts comments authored by anyone other than agentLogin.
 func countNonAgentComments(comments []Comment, agentLogin string) int {
 	n := 0