Set `stackql-deploy/{version}` User-Agent on the stackql binary download client

[jeffreyaven]

Intent

The stackql binary pull already routes via releases.stackql.io (app.rs STACKQL_RELEASE_BASE_URL), but the reqwest client sends no User-Agent, so this channel is blank in Analytics Engine. Add a stackql-deploy/{version} UA. One change covers both the GitHub Action and direct CLI use (same download_binary path, all platforms).

Change (src/utils/download.rs)

  let client = Client::builder()
+     .user_agent(concat!("stackql-deploy/", env!("CARGO_PKG_VERSION")))
      .timeout(std::time::Duration::from_secs(300))
      .build()

Acceptance

• proxy logs show UA stackql-deploy/<version> for stackql pulls on all platforms
• version matches CARGO_PKG_VERSION

Optional

src/commands/init.rs has a second reqwest client (template scaffolding from raw.githubusercontent.com). Out of scope for proxy attribution; apply the same .user_agent(...) there only if uniform branding is wanted.