Add noop instructions to repo-assist and regression-pr-shepherd workflows (#19567)

* Initial plan

* Add noop instructions to repo-assist and regression-pr-shepherd workflows

Reinforce noop requirement at key decision points:
- repo-assist: Command Mode exit path and Task FINAL fallthrough
- regression-pr-shepherd: When no eligible PRs exist

Fixes #17258 workflow failure (no safe outputs produced)

Agent-Logs-Url: https://github.com/dotnet/fsharp/sessions/ac165649-72fa-4613-bd06-fcdc9604f176

Co-authored-by: T-Gro <46543583+T-Gro@users.noreply.github.com>

---------

Co-authored-by: copilot-swe-agent[bot] <198982749+Copilot@users.noreply.github.com>
Co-authored-by: T-Gro <46543583+T-Gro@users.noreply.github.com>

Author: Copilot

.github/aw/actions-lock.json

@@ -5,15 +5,10 @@
       "version": "v8",
       "sha": "ed597411d8f924073f98dfc5c65a23a2325f34cd"
     },
-    "github/gh-aw-actions/setup@v0.67.1": {
+    "github/gh-aw-actions/setup@v0.67.4": {
       "repo": "github/gh-aw-actions/setup",
-      "version": "v0.67.1",
-      "sha": "80471a493be8c528dd27daf73cd644242a7965e0"
-    },
-    "github/gh-aw-actions/setup@v0.67.2": {
-      "repo": "github/gh-aw-actions/setup",
-      "version": "v0.67.2",
-      "sha": "03e31e064a68e8d5ad890c92f303cfb5a3536006"
+      "version": "v0.67.4",
+      "sha": "9d6ae06250fc0ec536a0e5f35de313b35bad7246"
     },
     "github/gh-aw/actions/setup@v0.67.2": {
       "repo": "github/gh-aw/actions/setup",