ci(publish): pack ANcpLua.Agents.Hosting.BitNet on alpha channel (#83)

PR #77 introduced the runtime package but never appended it to the
alpha-channel pack step in nuget-publish.yml, so every release since
v1.4.8 shipped the other Hosting.* siblings while BitNet stayed dark
on nuget.org. Consumers pin a BitNet version that does not exist and
restore fails with NU1101.

Channel choice: BitNet's csproj Description self-declares "Alpha-
channel" and wraps the experimental bitnet.cpp llama-server runtime,
so it rides the same -alpha.1 suffix as Hosting.OpenAI rather than
the -preview.1 train.

Trusted publishing is already covered by the existing owner-scoped
ANcpLua.Agents policy on nuget.org (no per-package policy needed).

Author: ANcpLua