Every outbound request went out as Go's default "Go-http-client/1.1",
which tells a node operator reading their algod access log, or the
GitHub release API, nothing about what is calling them. Send
"Nodekit v<version>" instead, built from the version the linker stamps
into main so that releases can be told apart.

The shared HttpPkg wrapper covers the release checks, the catchpoint
lookup, the short links and the upgrade download; the generated algod
client gets the header through a request editor, since api/lf.go is
overwritten by `make generate`.