Add new `--user-agent` flag to `chat`, `nsql` and `sql`

[eadgbear]

Document that the subcommands now support providing a custom User-Agent string using the --user-agent parameter

[claudespice]

The --user-agent CLI flag on spice chat, spice nsql, and spice sql no longer exists. In the current runtime (bin/spice), the User-Agent is auto-derived from the package version and host OS/arch in RuntimeContext::default_user_agent() (bin/spice/src/context.rs) and forwarded internally to spiced — it is not a user-configurable flag on the subcommands:

```rust
// bin/spice/src/context.rs
fn default_user_agent() -> String {
format!("spice/{} ({}; {})", env!("CARGO_PKG_VERSION"), /* os /, / arch */)
}
```

The subcommand args in bin/spice/src/commands/chat.rs expose --model, --temperature, --endpoint, and --headers — but no --user-agent. Custom HTTP headers can still be set with --headers 'Key:Value' if callers need to identify themselves.

Closing as not planned — the feature the issue asked to document is no longer present. If the expectation is that users should be able to override User-Agent on these subcommands, please file a new runtime issue in spiceai/spiceai first.