onlyworlds 0.3.0

• The door of every write (keel #299, D104). Client(..., agent=True) sends X-OW-Agent on every element write (create, upsert, patch, delete, edit_links, bulk: the operations keel's document declares it on; never on reads or patch_world), and keel records those writes as agent-made (changed_via: "declared"); keel stores that the header was there, never its value. Off by default, and off in any tool a person uses: it describes the caller. An agent seat's key counts as an agent without it.
• push never sends changed_by or changed_via, keel's new read-only record of the last write, for the reason it never sends created_by: keel drops them but still restamps the element, so a folder that holds them would rewrite elements for nothing.