Report work that ends after the skill timeout as llm.ErrTimeout

runOnce only mapped an expired context to llm.ErrTimeout when execute
returned an error, so a PostProcess that succeeded after the deadline was
reported as a success. Check the deadline after execution (context cause
or the clock, since the timer may not have fired yet) and fail such a run
with llm.ErrTimeout and a provider_error ai_runs row.

Adds a TestRunTimeout case with a skill whose PostProcess succeeds only
after its timeout.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

internal/ai/harness/guard_test.go

@@ -18,6 +18,17 @@ type timeoutSkill struct {
 
 func (s timeoutSkill) Timeout() time.Duration { return s.timeout }
 
+// slowSkill is timeoutSkill whose PostProcess succeeds only after its
+// timeout has passed.
+type slowSkill struct {
+	timeoutSkill
+}
+
+func (s slowSkill) PostProcess(out echoOutput) (echoOutput, error) {
+	time.Sleep(3 * s.timeout)
+	return s.timeoutSkill.PostProcess(out)
+}
+
 // blocked is one scripted Generate call that blocks until released.
 type blocked struct {
 	started chan struct{}
@@ -266,6 +277,24 @@ func TestRunTimeout(t *testing.T) {
 			t.Fatalf("rows = %+v, want one provider_error row naming the timeout", rows)
 		}
 	})
+	t.Run("work that succeeds late", func(t *testing.T) {
+		var f llm.Fake
+		cfg := testConfig(t, &f)
+		f.QueueGenerate(reply(`{"echo":"taco"}`))
+		s := slowSkill{timeoutSkill{timeout: 20 * time.Millisecond}}
+		out, err := Run(context.Background(), cfg, s, "week-1", echoInput{Word: "taco"})
+		if !errors.Is(err, llm.ErrTimeout) {
+			t.Fatalf("err = %v, want ErrTimeout", err)
+		}
+		if out != (echoOutput{}) {
+			t.Fatalf("out = %+v, want no output", out)
+		}
+		assertCalls(t, &f, 1)
+		rows := runRows(t, cfg.Store)
+		if len(rows) != 1 || rows[0].Status != string(StatusProviderError) || !strings.Contains(rows[0].Error.String, llm.ErrTimeout.Error()) {
+			t.Fatalf("rows = %+v, want one provider_error row naming the timeout", rows)
+		}
+	})
 	t.Run("waiting for a slot", func(t *testing.T) {
 		var f llm.Fake
 		cfg := testConfig(t, &f)