deps: point SDK dependency to crates.io v0.2.0

[collinjackson]

Summary

Point the Nexus Exchange SDK dependency from branch = "main" to the published crate version 0.2.0 on crates.io.

The SDK was published on 2026-06-23; this allows the CLI to build against the released version rather than tracking HEAD on the SDK repo.

Test plan

• cargo check completes successfully and resolves SDK v0.2.0 from crates.io
• CLI builds locally without errors

[collinjackson]

Blocked — do not merge yet, and the target version is wrong.

Repointing to 0.2.0 breaks the build. The published nexus-exchange 0.2.0 (latest on crates.io) is missing five APIs that main's CLI already uses, so cargo check fails with 5 errors:

• nexus_exchange::auth::AgentRegistration, nexus_exchange::EthSigner (imports — main.rs:18,20)
• Client::sign_in, Client::register_agent (main.rs:596,721)
• Config::with_user_agent (cli.rs:205)

These are the wallet-signed auth + User-Agent APIs, added to the SDK after 0.2.0. They live in nexus-exchange-rs main and ship in v0.3.0, which is still an open release PR (nexus-xyz/nexus-exchange-rs#47, not yet published). main's CLI only compiles today because it tracks the SDK via git branch = "main".

Plan: once rs#47 (v0.3.0) is published to crates.io, repoint to nexus-exchange = "0.3.0" (not 0.2.0), regenerate Cargo.lock, confirm cargo check is clean. Tracked in ENG-4212.

Keeping this PR as the one to carry that change. Closing #8 (the older duplicate of the same repoint) in favor of this one.