Tie nudge validity to the on-task stretch

A soft nudge advisory belongs to one continuous on-task stretch in an allowed
app. Guard it with an on-task-stretch epoch (advanced on session reset and on
any non-on-task-match observation) instead of a session-only counter, and clear
the advisory on leaving the allowed app, so a stale 'Heads up' never resurfaces
after a drift episode.

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

internal/session/session_test.go

@@ -693,11 +693,26 @@ func TestNilNudgerStaysSilentOnTask(t *testing.T) {
 	}
 }
 
-func TestNudgeSurvivesInterleavedDriftJudgment(t *testing.T) {
+func TestNudgeClearedOnLeavingAllowedApp(t *testing.T) {
 	c, _ := newTestController(t)
-	fn := &fakeNudger{msg: "kept", gate: make(chan struct{})}
+	c.SetNudge(&fakeNudger{msg: "drifting within editor"})
+	startActive(t, c, []string{"code"})
+	c.RecordWindow(obs("code", "a.go"))
+	c.RecordWindow(obs("code", "b.go"))
+	waitNudge(t, c, "drifting within editor")
+	// Leaving the allowed app ends the on-task stretch and voids the advisory.
+	c.RecordWindow(obs("firefox", "YouTube"))
+	if st := c.State(); st.Drift == nil || st.Drift.Nudge != "" {
+		t.Fatalf("advisory must clear when leaving the allowed app: %+v", st.Drift)
+	}
+}
+
+func TestNudgeInFlightDiscardedAfterDriftEpisode(t *testing.T) {
+	c, _ := newTestController(t)
+	now := time.Now()
+	c.SetClock(func() time.Time { return now })
+	fn := &fakeNudger{msg: "stale", gate: make(chan struct{})}
 	c.SetNudge(fn)
-	c.SetDriftJudge(&fakeJudge{verdict: ai.Verdict{OnTask: false, Reason: "off-task"}})
 	startActive(t, c, []string{"code"})
 	c.RecordWindow(obs("code", "a.go"))
 	c.RecordWindow(obs("code", "b.go")) // nudge launches, blocks on gate
@@ -705,11 +720,13 @@ func TestNudgeSurvivesInterleavedDriftJudgment(t *testing.T) {
 	for time.Now().Before(deadline) && atomic.LoadInt32(&fn.calls) == 0 {
 		time.Sleep(5 * time.Millisecond)
 	}
-	// An interleaved drift judgment bumps driftGen; the nudge must NOT be
-	// discarded by it — it is guarded by session identity, not driftGen.
-	c.RecordWindow(obs("firefox", "YouTube"))
-	close(fn.gate) // nudge returns within the same session
-	waitNudge(t, c, "kept")
+	c.RecordWindow(obs("firefox", "YouTube")) // ends the on-task stretch -> epoch++
+	close(fn.gate)                            // in-flight nudge returns now
+	c.RecordWindow(obs("code", "c.go"))       // back on task (debounced; no new nudge)
+	time.Sleep(50 * time.Millisecond)
+	if st := c.State(); st.Drift == nil || st.Drift.Nudge != "" {
+		t.Fatalf("in-flight nudge from an ended stretch must not surface: %+v", st.Drift)
+	}
 }
 
 func TestStaleNudgeDiscardedAfterEnd(t *testing.T) {