This, assumes the vendor acts in good faith
Responsible disclosure does not assume the vendor acts in good faith. Usually the disclosure period is around 90 days before the vulnerability is released, fixed or not (although this is negotiable with a good faith vendor).
Forks etc. could have been informed privately first too if possible.
amateurs now have access to tools they should not, and WILL forgo proper standardized communication channels to disclose issues
This is not a good argument. Undisclosed zero days in the wild have always been part of the threat model. Amateurs with LLMs or not, a large percentage of vulnerabilities are not disclosed responsibly and are only fixed after damage has been done. Putting people and their personal information at risk because you want to make a point about the dangers of zero days (which everyone is already aware of) is woefully unethical.
Not everyone is privileged enough to afford security courses, and standardized education.
That doesn't mean we should abandon these things. The vendor can report the CVE too. Or anyone else with an interest in it. It doesn't have to be the untrained amateur grey hat asking Claude for vulns. A malicious threat actor exploiting a system doesn't report it either. The community benefits from skilled people handling things properly. Pretending that it doesn't because most people don't have those skills is silly.
They don't need to have one.
You can report it here: https://cveform.mitre.org/
Use the CNA-LR since I don't think they have a CNA.
You were probably trying to do the right thing disclosing, just know that there is a better process for it (even if you think the devs are asshats, it's good to do it like that for the community who aren't).
Even if it only affects admins, that includes admins of forks etc.
I'm sure there's probably more vulnerabilities to find.