feat(agent): auto-complete tasks when all plan steps are terminal

The #1 remaining model reliability gap: the agent does the work (proposes
plan, executes all steps, writes back) but forgets to call complete_task,
leaving the session stuck in 'executing'. The eval showed 3/8 failures
with this pattern.

Fix: autoCompleteIfPlanDone — a structural safety net that fires at both
chat exit paths (normal completion + maxIterations). If the session has a
goal, the agent didn't call complete_task, and ALL plan steps are in a
terminal state (done/failed/replaced/skipped/blocked), auto-complete with
the agent's final text as the summary. Mirrors autoCompleteTrivialTask
but for structured tasks where the work is provably done.

Also: bump maxLLMRetries from 2 to 3 (complex multi-turn flows benefit
from one more retry on empty responses).

cmd/nomos/tasks.go

@@ -357,3 +357,39 @@ func (a *agent) autoCompleteTrivialTask(ctx context.Context, sessionID, response
 		slog.Error("nomos: auto-complete trivial task failed", "session", sessionID, "error", err)
 	}
 }
+
+// autoCompleteIfPlanDone is the structural safety net for "the agent did the
+// work but forgot to call complete_task" — the #1 remaining model reliability
+// gap after D.1's writeback gate. After a turn ends, if the session has a goal,
+// the agent never called complete_task this turn, and ALL plan steps are in a
+// terminal state (done/failed/replaced), auto-complete the task. This mirrors
+// autoCompleteTrivialTask but for structured tasks where the work is provably
+// done — the model just didn't close the loop.
+func (a *agent) autoCompleteIfPlanDone(ctx context.Context, sessionID, responseText string) {
+	if a.store == nil || sessionID == "" || sessionID == "ephemeral" {
+		return
+	}
+	// Only auto-complete if the session is still executing (not already
+	// terminal — complete_task or a prior auto-complete already ran).
+	sess, err := a.store.getSession(ctx, sessionID)
+	if err != nil || sess.Status != "executing" {
+		return
+	}
+	if !a.store.allPlanStepsTerminal(ctx, sessionID) {
+		return
+	}
+	summary := strings.TrimSpace(responseText)
+	summary = strings.SplitN(summary, "\n", 2)[0]
+	const maxLen = 120
+	if len(summary) > maxLen {
+		summary = summary[:maxLen] + "…"
+	}
+	if summary == "" {
+		summary = "All plan steps completed."
+	}
+	if err := a.store.completeTask(ctx, sessionID, "success", summary); err != nil {
+		slog.Error("nomos: auto-complete plan-done task failed", "session", sessionID, "error", err)
+	} else {
+		slog.Info("nomos: auto-completed task — all plan steps terminal but agent didn't call complete_task", "session", sessionID)
+	}
+}