Add ServerConfig for arc.toml server configuration

Introduces server_config.rs with a TOML-based ServerConfig struct
(version + url) following the same patterns as TaskConfig.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

crates/arc-workflows/src/cli/mod.rs

@@ -3,6 +3,7 @@ pub mod cli_backend;
 pub mod run;
 #[cfg(feature = "server")]
 pub mod serve;
+pub mod server_config;
 pub mod task_config;
 pub mod validate;
 

crates/arc-workflows/src/cli/server_config.rs

@@ -0,0 +1,85 @@
+use std::path::Path;
+
+use anyhow::{bail, Context};
+use serde::Deserialize;
+
+const SUPPORTED_VERSION: &str = "1";
+
+#[derive(Debug, Deserialize)]
+#[serde(deny_unknown_fields)]
+pub struct ServerConfig {
+    pub version: String,
+    pub url: String,
+}
+
+pub fn load_server_config(path: &Path) -> anyhow::Result<ServerConfig> {
+    let contents =
+        std::fs::read_to_string(path).with_context(|| format!("Failed to read {}", path.display()))?;
+    parse_server_config(&contents)
+}
+
+fn parse_server_config(contents: &str) -> anyhow::Result<ServerConfig> {
+    let config: ServerConfig =
+        toml::from_str(contents).context("Failed to parse server config TOML")?;
+
+    if config.version != SUPPORTED_VERSION {
+        bail!(
+            "Unsupported server config version {}. Only version {SUPPORTED_VERSION} is supported.",
+            config.version
+        );
+    }
+
+    Ok(config)
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn parse_valid_config() {
+        let toml = r#"
+version = "1"
+url = "http://localhost:3000"
+"#;
+        let config = parse_server_config(toml).unwrap();
+        assert_eq!(config.version, "1");
+        assert_eq!(config.url, "http://localhost:3000");
+    }
+
+    #[test]
+    fn unsupported_version_rejected() {
+        let toml = r#"
+version = "2"
+url = "http://localhost:3000"
+"#;
+        let err = parse_server_config(toml).unwrap_err();
+        assert!(
+            err.to_string().contains("Unsupported server config version 2"),
+            "unexpected error: {err}"
+        );
+    }
+
+    #[test]
+    fn unknown_fields_rejected() {
+        let toml = r#"
+version = "1"
+url = "http://localhost:3000"
+extra = "nope"
+"#;
+        assert!(parse_server_config(toml).is_err());
+    }
+
+    #[test]
+    fn missing_fields_rejected() {
+        let no_url = r#"
+version = "1"
+"#;
+        assert!(parse_server_config(no_url).is_err());
+
+        let no_version = r#"
+url = "http://localhost:3000"
+"#;
+        assert!(parse_server_config(no_version).is_err());
+    }
+}