feat(client): testjam about + AboutResource SDK

[jamofer]

Summary

• TestjamClient.about.get() returns a frozen ServerAbout(version, ref, commit, built_at) dataclass. Mirrors the server response. No auth required.
• New testjam about CLI command prints the client version side-by-side with the connected server's version/ref/commit/built_at. Honours --json.
• On network errors or missing profile URL, the server fields collapse to unknown and server_status reads unreachable — the command still exits 0 so it remains a safe smoke probe.
• SDK tests use httpx.MockTransport; CLI tests patch runtime.build_client. Self-contained — no backend route needed in the test process.

Requires the /api/v1/about route from PR #50 to be live before the CLI surfaces real values against a deployed server.