Prebuilds: accept unslothai/llama.cpp PRs in pr-set.json

[oobabooga]

Expands scripts/unsloth/pr-set.json to accept pins from unslothai/llama.cpp PRs, not just ggml-org/llama.cpp. This lets fork-only changes ship in the nightly prebuilds without waiting for an upstream PR.

How it works

• A pin is still a commit URL, now from either repo (and only those two): https://github.com/{ggml-org,unslothai}/llama.cpp/pull/<n>/commits/<sha>.
• The resolve job validates each pin against its home repo (open state, pin-membership check) and fetches and merges it from that repo. The base tree is still the aged upstream b#### tag.
• Pin entries carry a repo field, and the -mix-<hash> tag now hashes repo#number:sha, so the same PR number in the two repos can't collide. Side effect: the first nightly after this lands rebuilds the current pin set once under a new tag, same content.
• Release notes spell out the repo for fork pins (unslothai/llama.cpp#16) and keep the plain #24423 form for upstream ones.
• The pr-set lint mirrors the same URL and validation changes.

Hardenings and docs

• A nonexistent PR number (easy to hit against the fork's small PR numbers) now fails naming the offending entry instead of dying with a bare gh: Not Found. The lint reports it and keeps checking the remaining entries.
• pr-set.json documents two fork-pin gotchas: a PR branched off fork master merges that ancestry into the tree, and merging a fork PR into master drops its content from the nightly (the build tree is the upstream tag plus pins), so fork pins must stay open until the change lands upstream.

Verification

Simulated the full resolve flow locally on base b9902. The mixed set (the 3 current ggml-org pins plus #16) merges cleanly end to end, with correctly labeled merge commits and intact build-info stamping; #11 and #15 conflict with the current base and abort with the reorder-or-drop message as intended. Exercised the URL validation against real PRs from both repos plus malformed and wrong-repo URLs. Also exercised the 404 paths in both workflows against the live API. Nothing downstream consumes the manifest's merged_prs, so the added repo field is additive.

[oobabooga]

@codex review

[oobabooga]

/gemini review

[chatgpt-codex-connector]

Codex Review: Didn't find any major issues. Breezy!

Reviewed commit: ef29dd8bad

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[oobabooga]

/gemini review

[chatgpt-codex-connector]

💡 Codex Review

Here are some automated review suggestions for this pull request.

Reviewed commit: 5eed57e071

ℹ️ About Codex in GitHub

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• Mark a draft as ready
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