variable: call the new /v1/variables routes

[hbrooks]

Summary

• Points the three variable commands at /v1/variables instead of /v1/sandboxes/variables.
• The API side moved in ellipsis-dev/ellipsis#6002 and is already live in prod: the new path serves, the old one 404s. This is the client half of that move.
• No fallback to the old paths. Variables are pre-customer and this CLI is their only client.
• The variable verb, its aliases, and its output are unchanged. Only the URLs it calls and the API: lines in --help move.
• Bumps @ellipsis-dev/sdk to 0.5.0 (published from ellipsis#6003), which adds missing_sandbox_variables to AgentSessionExitStatus. Nothing here narrows to that union yet, since exit_status is still typed as a plain string, but the CLI should track the published SDK rather than drift behind.

Test plan

• bun run typecheck
• bun run test — 375 passed
• bun run build
• bun build src/cli.tsx --compile then --version (matches the CI compile-smoke step)
• agent variable list --help shows API: GET /v1/variables
• Ran the compiled binary against live prod: agent variable list returns the account's stored variable, confirming the new route end-to-end
• Re-ran all of the above after the SDK bump