[omartelo]

Summary

The Pulls screen's Files tab drew a changed image as "Binary file" and offered the Markdown preview only for an added file, because /blob and ReadFile read only what the clone holds. Both now reach a pull request's commits.

• Images: /blob reads an object the clone lacks from GitHub's contents API, the way FileLines already does; a commit the clone holds is always read locally. The before side stands on the new project.PullRequestMergeBase RPC: the tips come from GitHub, and the merge base is computed with git merge-base when the clone holds both, else asked of the compare API. baseRefOid is not used, since it is the base branch's tip and goes wrong once the base moves.
• Rate limit: the window asks the merge base once per pull request head, shared by every image card (mergeBaseMemo), and asks again only for a new head or after a failure.
• Markdown: an edited file there gets the preview toggle, read at the head through the new project.ReadFileAt (local object first, GitHub otherwise, ReadFile's size and binary refusals).
• docs/ceilings.md and CHANGELOG.md move with it.

Two native tests changed because their contract changed: TestBinaryFilesOpenWhenTheyHaveAPreview asserted an image in a pull request starts collapsed since the clone held neither side, and TestMarkdownPreviewIsOfferedWhereTheNewVersionIsReadable asserted an edited file in a pull request has no preview.

Test plan

• Backend: gofmt, go vet ./..., go test ./... (new tests for the GitHub fallback, local-only reads, size cap, merge base local/compare/unreadable, ReadFileAt)
• Native: gofmt, go vet ./..., go test ./...; mergeBaseMemo tests under -race -count=20
• Against docs: show the native window on the social card #894 on GitHub: merge base and both sides of docs/media/og.png read from this clone and from an empty clone with only the remote, same bytes on each path