chore(changeset): add the conventional-commits bang to the breaking declaration

`breakingDeclaration` reads three independent signals and the union is what
CI records; the banner alone already declared it. The bang is added so the
summary line carries the declaration too, matching the other declared-breaking
changesets in stock.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_012zTkyNHJ7TkuN2oXtP5x37

Author: claude

.changeset/report-schedule-timezone-guard.md

@@ -2,7 +2,7 @@
 "@objectstack/plugin-reports": minor
 ---
 
-fix(plugin-reports): a non-member schedule `timezone` no longer discards the cron expression, and a schedule already holding one stops instead of firing on a cadence nobody asked for (#16291)
+fix(plugin-reports)!: a non-member schedule `timezone` no longer discards the cron expression, and a schedule already holding one stops instead of firing on a cadence nobody asked for (#16291)
 
 **BREAKING** for a deployment that already stores a report schedule with a cron expression and a `timezone` that is not an IANA member. Such a schedule is delivering today, on the wrong cadence; after this change it does not deliver at all until a human corrects the zone. It ships as `minor` under the lockstep launch-window convention (`scripts/check-changeset-no-major.mjs` refuses `major`); the version number is not the signal here, this entry is.