Cover thin remote clone branch metadata

[adamziel]

What it does

Adds a focused COW E2E check for forkpress remote clone --branch.

The test uses a local fake rsync to prove the command creates a boot cache that skips wp-content/uploads/ and cache files by default, then immediately creates a branch from that cache.

Rationale

remote clone should make a remote site bootable without pulling large cold-start data like uploads. It also needs to use the normal branch-birth path so the new branch has ID bands, a DB merge base, and a filesystem merge base before the first write.

Implementation

In the existing FORKPRESS_E2E_ONLY=remote-cache slice, the test now:

• creates a local remote-like WordPress root with an upload and cache file
• shadows rsync with a fake executable that honors --exclude
• runs forkpress remote clone thin-prod --branch remote-thin-branch
• asserts the remote cache and branch include boot files but not uploads/cache files
• writes through WordPress on the cloned branch and verifies the inserted ID lands inside the branch band
• checks both cow/merge/bases/remote-thin-branch.sqlite and cow/merge/file-bases/remote-thin-branch.json exist

Testing instructions

Ran locally:

bash -n tests/cow/e2e.sh
git diff --check
FORKPRESS_E2E_ONLY=remote-cache tests/cow/e2e.sh target/x86_64-unknown-linux-musl/debug/forkpress

The focused E2E slice passed. Local COW storage fell back to file-copy because loop devices are not available in this environment.

[adamziel]

Rebased onto current trunk after #324. Local verification after rebase:\n\n- bash -n tests/cow/e2e.sh\n- git diff --check\n\nThe focused remote-cache E2E slice was already run locally before this rebase and passed.