Report server version in rossoctl version

[esnible]

Summary

• rossoctl version now also prints the connected server's version (via GET /auth/config), falling back to "unknown" if unreachable.
• cortex serve's /auth/config now reports its own build version, sourced from a shared buildinfo.Version (ldflags target moved from cmd.version).

Test plan

• go build ./..., go vet ./..., go test ./...
• Manual: rossoctl version against live server and against unreachable server
• Manual: cortex serve + curl /auth/config shows injected version

Assisted-by: Claude

Signed-off-by: Ed Snible

[mrsabath]

Summary

Clean, well-scoped feature: rossoctl version now also prints the connected server's version from GET /auth/config, and cortex serve reports its own build version via a shared buildinfo.Version.

I verified the version field is consistent end-to-end across three surfaces:

• CLI client model (internal/apiclient.AuthConfig.Version)
• cortex serve (internal/serve.AuthConfig.Version)
• the real rossoctl Python backend, which already returns version in /auth/config (AuthConfigResponse.version in rossoctl/rossoctl auth.py).

So rossoctl version reports the actual backend version against a live server, not only against cortex serve, and falls back to "unknown" gracefully when the server is unreachable or omits the field. Moving version out of cmd into buildinfo is the right call to avoid an import cycle from internal/serve, and the Makefile ldflags target was updated to match.

Good test coverage: three CLI cases (reachable / server-omits-version / unreachable) plus the updated serve test asserting the new field.

Areas reviewed: Go (cmd/serve/apiclient/buildinfo), tests, Makefile, cross-repo API contract
Commits: 1, signed-off (DCO passes)
CI status: passing (DCO + test)

LGTM. One optional cosmetic nit inline.

[mrsabath]

nit (optional, non-blocking): BUILDINFO_PKG := is longer than the column BINARY/PKG/CMD_PKG were padded to, so its := no longer lines up with the three above it, and the VERSION/COMMIT/DATE block below still uses the older narrower alignment. Purely cosmetic (make doesn't care). Either widen all of them to match BUILDINFO_PKG, or drop the manual padding entirely.