test(e2e): content protocol, drafts and publish over GitHub

[tlgimenes]

Part of the v8 Studio stack, split from #7728, #7770 and #7766 (closed). Base: #7796. The top of the stack, #7843, has the same code as those three PRs combined.

Adds the end-to-end spec for the draft, publish and release loop on the GitHub backend. The GitHub stub gains git-compatible blob ids, sha-based compare and the RepoCommits GraphQL query; fast-preview projects can carry a siteSlug.

Merge after the API, daemon and web stacks (and after #7841 and #7842). This PR's own diff is only its 3 files.

The tree of this branch is identical to feat/blocks-v8-hosted (a68e1ac): git diff --stat origin/feat/blocks-v8-hosted origin/split/44-content-protocol-github-e2e is empty, and both trees hash to 42a84aaa3.

What to look at

• The end-to-end draft, publish and release loop, and the stub additions it needs.

Stack

e2e track: previous: #7842 · next: none (top of the whole stack)

Business-language copy: the publish spec also checks releases/current { head: true } (Publish's Try again) and its title drops CDN/Make current wording.

🤖 Generated with Claude Code

https://claude.ai/code/session_01WNwbSEePYNcY5YCgqZURig

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Summary by cubic

Adds end-to-end coverage for the content protocol's draft, publish, and release loop on the GitHub backend, plus the stub and fixture extensions the spec needs. This PR is test-only: its diff is the new spec and two fixture files.

What's covered

• Protocol conformance, org-flag and session gating, and CDN draft saves that never touch git.
• Publish commits the draft to main and releases it; Make current and Publish's "Try again" switch the live revision via latest.json, with every write re-stamped to the present.
• The ?__draft= pointer with ETag revalidation, site tokens, and v7/v8 detection (only a committed schema with "blocksMajor": 8 is a v8 site).
• The GitHub stub gains git-compatible blob ids, sha-based compare, and a RepoCommits GraphQL query; fast-preview projects can carry a siteSlug to link the project to its site.

Merge after the API, daemon, and web stacks (and after the preceding e2e PRs in this stack).

^{Written for commit 9a05e81. Summary will update on new commits.}