Create pf-prerelease-check agent

[patternfly-jira-sync]

Agent that validates a PF release candidate against downstream product repos. Installs the RC, runs builds and tests, reports compatibility.

AC:

• Agent exists in pf-internal plugin

• Installs a PF RC into a target product repo

• Runs build + test suite and reports results

---

Jira Issue: PF-4187