ci: tell Claude it MUST call Write tool, not emit JSON as text

Previous run: allowedTools=Read,Write granted, permission_denials_count=0,
Claude reported success in 3 turns — but the assistant message contained
the JSON inline ("Quiet day" payload composed correctly) instead of
invoking the Write tool. POST to Slack then errored on the absent file.

Stronger imperative + warning: spell out the Write call signature and
state explicitly that printing JSON does not count. If this still
doesn't trigger Write, next iteration switches to parsing the action's
execution_file output and writing the payload ourselves.

Author: Jammy2211

.github/workflows/morning_status.yml

@@ -148,8 +148,16 @@ jobs:
             release bump (if any) first, then merged PRs, then direct
             pushes.
 
-            Then write `slack_payload.json` in the repo root, with this
-            exact shape:
+            **You MUST invoke the Write tool to create the file at the path
+            `slack_payload.json` (relative to the current working directory).**
+            Do not output the JSON in your text response. Do not describe
+            the file. Do not preview it. Just call Write with `file_path:
+            "slack_payload.json"` and `content:` set to the JSON below.
+            The next workflow step reads that file from disk and fails the
+            run if it is missing — printing the JSON without writing it
+            does NOT count.
+
+            The exact JSON shape to write:
 
               {
                 "text": "PyAuto update — <uk_date>",