Write one task file [write]

The body is the file's bytes (16 MiB at most; 413 file_too_large). The write becomes the current file. base_sha256 names what the caller last saw at the path (empty for nothing); when something else is current the default write is still made, both versions are kept, and the response says conflict: true. With require_base_match=true, a base is required and a mismatch instead returns 409 revision_conflict without updating the file. A task's space has no total-size or file-count limit. Sessions of the task pick the file up at the start of their next turn.