codeSplitters branches: carry binary files (a blob-backed branch copy) #446

Split out of #440. Branches can now delete files, so upstream deletions (binary files included) carry into mirror branches, merge into main and push back up.

What's left: a binary file added or changed on an upstream branch still can't come over. A branch's copy of a file is a Durable Object of text lines, so take in apps/codesplitters/src/mirror.ts lists binary files as skipped and pushBranch never sends them.

Needs a blob-backed branch copy (e.g. the bytes in git or R2, recorded on branch_files), which the review shows as "binary, changed", the merge writes into main's git, and pushBranch puts in the tree.