feat(tasks): phase 4 — structured plan steps (set_goal/propose_plan/update_plan_step)

Gives a task a legible, live-advancing plan via three more nomos-local tools:

- set_goal(goal): records the task goal, status → planning, emits goal.set.
- propose_plan(steps[]): persists ordered steps (clean replace for v1 — a
  revision starts a new list), status → executing, emits plan.proposed with
  the persisted steps (id+seq) so the panel can address them.
- update_plan_step(seq, status, execution_id?): advances a step, stamping
  started_at/finished_at, emits plan.step.started/finished. Anchors the event
  to the step's target entity when it has one.

Belt-and-suspenders: when an execution linked to a step reaches a terminal
state, the api auto-closes the step (closePlanStepForExecution in
emitExecutionEvent) and emits plan.step.finished — so the board stays honest
even if the agent forgets to close a step it started.

Verified end-to-end: a goal-driven task fired goal.set → plan.proposed →
2× step.started/finished → task.status on the SSE stream; both steps persisted
done with start/finish timestamps; status progressed planning→executing→done.

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

internal/httpapi/phase3.go

@@ -231,6 +231,32 @@ func emitExecutionEvent(ctx context.Context, pool *db.Pool, execID uuid.UUID, st
 		severity = "warning"
 	}
 	_ = observability.Event(ctx, sqlcgen.New(pool), "execution."+status, &execID, severity, "actuator", "", detail)
+	if status == "completed" || status == "failed" || status == "cancelled" {
+		closePlanStepForExecution(ctx, pool, execID, status)
+	}
+}
+
+// closePlanStepForExecution auto-closes a task plan step whose linked execution
+// just reached a terminal state, so the task board advances even if the agent
+// doesn't call update_plan_step itself (belt and suspenders — the agent links
+// the step to the execution when it starts it; the api finishes it here). Emits
+// plan.step.finished correlated to the step's session. No-op for the vast
+// majority of executions, which aren't plan steps.
+func closePlanStepForExecution(ctx context.Context, pool *db.Pool, execID uuid.UUID, execStatus string) {
+	stepStatus := "done"
+	if execStatus == "failed" || execStatus == "cancelled" {
+		stepStatus = "failed"
+	}
+	var stepID, sessionID string
+	var seq int
+	if err := pool.QueryRow(ctx, `
+		UPDATE session_plan_steps SET status = $2, finished_at = now()
+		WHERE execution_id = $1 AND status NOT IN ('done', 'failed', 'skipped')
+		RETURNING id::text, session_id::text, seq`, execID, stepStatus).Scan(&stepID, &sessionID, &seq); err != nil {
+		return // no matching open step
+	}
+	_ = observability.Event(ctx, sqlcgen.New(pool), "plan.step.finished", &execID, "info", "actuator", sessionID,
+		map[string]any{"step_id": stepID, "seq": seq, "status": stepStatus, "execution_id": execID.String()})
 }
 
 func executeApprovedAction(ctx context.Context, pool *db.Pool, execID uuid.UUID, targetSlug string, actionStr string) {