fix(standards): restore schedule trigger and align AGENTS block with main

- Restores the `schedule: cron '0 */4 * * *'` trigger that drifted out
  during branch divergence (main added it after the branch forked)
- Restores the fuller AGENTS MUST NOT comment that explains why all
  triggers are protected (matches main's authoritative version)
- Preserves all branch improvements: SHA-pin rationale, adoption
  guidance directing to standards/, and `contents: write` permission

This aligns the standards template's trigger/comment sections with
main's version, reducing conflict scope when the branch is rebased.

Co-authored-by: Don Petry <don-petry@users.noreply.github.com>

Author: github-actions[bot]

standards/workflows/dependabot-rebase.yml

@@ -12,11 +12,13 @@
 #     this internal reusable workflow. Downstream repos pin to a specific
 #     commit SHA and bump deliberately. To look up the current SHA:
 #     gh api repos/petry-projects/.github/branches/main --jq '.commit.sha'
-#   • You MUST NOT change: trigger event, the concurrency group name,
-#     the explicit secrets block, or the job-level `permissions:` block —
-#     reusable workflows can be granted no more permissions than the calling
-#     job has, so removing the stanza breaks
-#     the reusable's gh API calls.
+#   • You MUST NOT change: the concurrency group name, the explicit secrets
+#     block, or the job-level `permissions:` block — reusable workflows can be
+#     granted no more permissions than the calling job has, so removing the
+#     stanza breaks the reusable's gh API calls. Do not remove any trigger
+#     (`push` keeps the self-sustaining chain; `schedule` is the safety net
+#     when no PR merges have occurred recently; `workflow_dispatch` allows
+#     manual queue flushes).
 #   • If you need different behaviour, open a PR against the reusable in the
 #     central repo.
 # ─────────────────────────────────────────────────────────────────────────────
@@ -35,6 +37,8 @@ on:
   push:
     branches:
       - main
+  schedule:
+    - cron: '0 */4 * * *' # every 4 hours — safety net when no pushes to main trigger the chain
   workflow_dispatch: # allow manual trigger to flush Dependabot PR queue
 
 concurrency: