ci: add the required gate

[P4suta]

Adds required, the one status check every P4suta repository requires once the shared ruleset is applied.
It waits until every other check on the head commit has finished, then fails if any of them failed, so the ruleset never has to list this repository's own job names.

[coderabbitai]

Important

• 🔍 Trigger review

This repository does not receive automatic reviews because it has fewer than 10 stars.

⚙️ Run configuration

Configuration used: Organization UI

Review profile: CHILL

Plan: Advanced

Run ID: d38e73dc-e283-4780-9509-607b29dfda5e

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[P4suta]

Closing: the polling gate held a runner while it waited, which starves the account's CI when many pull requests open at once. Each repository gets a required aggregate job in its own CI instead.