refactor(agent): rename WRITE_BUDGET to WRITE_TIMEOUT

The constant bounds one HID++ call: every use is
`tokio::time::timeout(WRITE_BUDGET, ..)`, and each call gets the full five
seconds. A budget in this codebase is what `server.rs`'s `Budget` is, an
allowance shared across attempts and spent down. A duration that bounds a
single wait is a `_TIMEOUT`.