Attaching a local folder to a cloud session: file sync flow and earlier link preparation

What

A cloud session is a Claude Code session running on Anthropic's servers that can run tools on your own computer through a link. This release adds and changes several pieces of how a local machine attaches to such a session:

• New code lets the SDK host (a program that embeds Claude Code) attach a local git checkout to a running cloud session and start file sync, which keeps the local folder and the session's copy in step.
• It first checks the folder: it must be the top of a git repository, use a supported git layout, and be a checkout of the same repository the session works in. It then asks you through a sync-mode dialog, which must be one the host has declared it can show. Last, it checks that the folder is on the session's branch and that its commit is in the recent history of that branch.
• If every check passes, sync is armed and starts the next time Claude runs something on that computer. Each refusal has its own message, for example "File sync was not started: this folder sits inside a larger git repository."
• The only caller is the headless remote attach path. It runs when the session has no directory-sync handle and directory sync reports not_armed_here, and it then loads startAttachSync and starts it.
• The flow is controlled by the gate tengu_violin_soundpost, whose built-in default is off, and fails early with "flag_off" when the gate check does not pass. The flag server returned off for this site's account and for the anonymous baseline, but no reading has been taken under this release yet.
• In serve mode, attach now starts preparing the link while it reads the stream position, instead of afterwards, and passes the result to linkForServing along with handsKeyToLaterReaders. This is controlled by linkPreparationOverlapOn, which falls back to the gate tengu_violin_bassbar, whose built-in default is on. Nothing has been read about that gate yet.
• Linking now accepts localIdentity, prepared and handsKeyToLaterReaders, and the link step records which proof form it used (proof) and whether it retried with the v1 form (proof_v1_retry).

Why

These pieces point to attaching an existing local checkout to a cloud session and keeping the two in sync, with link setup overlapping other work at attach time. Whether the file sync flow runs for you depends on server-controlled gates.