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/agent.go

@@ -415,6 +415,14 @@ func (a *agent) chatWith(ctx context.Context, sessionID, message, systemInject s
 			if !sawSetGoal && !sawCompleteTask {
 				a.autoCompleteTrivialTask(ctx, sessionID, msg.Content)
 			}
+			// Safety net: if the agent called set_goal (structured task)
+			// but didn't call complete_task, and all plan steps are
+			// terminal, auto-complete. The model often does the work but
+			// forgets to close the loop (confirmed live: the #1 remaining
+			// model reliability gap after D.1).
+			if !sawCompleteTask {
+				a.autoCompleteIfPlanDone(ctx, sessionID, msg.Content)
+			}
 			emit(agentEvent{Type: "done", Data: map[string]any{
 				"session_id":     sessionID,
 				"usage":          acc.Usage,
@@ -566,6 +574,9 @@ func (a *agent) chatWith(ctx context.Context, sessionID, message, systemInject s
 		summary = "I hit this turn's step limit while working. I've done a lot but couldn't wrap up cleanly — ask me for a status update and I'll summarize the current state."
 	}
 	emit(agentEvent{Type: "text", Data: summary, SessionID: sessionID})
+	if !sawCompleteTask {
+		a.autoCompleteIfPlanDone(ctx, sessionID, summary)
+	}
 	emit(agentEvent{Type: "done", Data: map[string]any{
 		"session_id":     sessionID,
 		"correlation_id": correlationID,