Merge origin/main into paul/ssh-protocol-v2

Resolve the CHANGELOG conflict by keeping both Unreleased entries.

TestSSHRPCProtocolEnv used a shim that sends no ref advertisement. Since #128, PostRPCStreamBody waits for the advertisement before sending the request, so the test hung. Use newSSHShimEnvWith(t, "0000") as main's streaming tests do.