Stream attachments end to end, ship files to a remote session's machine, and write up the transcript work

Uploads no longer sit whole in memory anywhere: the phone writes the
multipart body chunked as it reads the picked file, and the server writes
each chunk to a `.part` file under the session and renames it when whole.
The per-request cap is 4 GB and bounds disk, not memory.

A file attached to a session on another machine is copied there in the
same request: one ssh invocation takes the bytes on stdin into the
setup's `attachmentsDir` (new, optional, on the machine form and in the
config), else the session's cwd, else the login home, and answers with
`pwd -P`, which is recorded beside the file as `<name>.remote` and is the
path the driver tells the CLI. A failed copy fails the upload and says
why, so no message ever names a file that is not there. The host keeps
its copy so transcripts can reference and fetch it. Measured against the
Gentoo test guest: a 40 MB file shared from the phone arrived there byte
for byte. The tilde in that setting is the remote home, so it is not
expanded on the server the way other setup paths are.

TRANSCRIPT_RENDERING.md records the week of transcript work -- the
measurements behind each decision, the harness, what was rejected, and
what to do next -- so a new session can start from it.

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>

server/src/usage.rs

@@ -423,6 +423,7 @@ mod tests {
                 port: None,
                 identity_file: None,
                 options: vec!["ConnectTimeout=1".to_string()],
+                attachments_dir: None,
             }),
             providers: vec![crate::config::ProviderConfig {
                 name: "claude-cli".to_string(),