fix(registry): un-stub brush tab completion on wasi

[NathanFlurry]

• Tab completion in the wasi brush build returned an empty candidate list ("NO RECORDS FOUND" on every Tab) because the wasi patch stubbed the completion entry points — upstream drives its async completion generator with tokio::runtime::Handle::current().block_on(...), and the single-threaded wasi tokio build has no runtime to block on.
• The stubs are replaced with a bounded noop-waker poll loop: the completion future only performs synchronous VFS walks and uncontended lock acquisition, so it resolves on the first polls; the bound bails out to no-completions if a future ever parks on a waker (which a single thread could never wake). Applies to both the reedline and basic input backends.