Accept story 30: Worktree-based agent orchestration

Add git worktree isolation for concurrent story agents. Each agent now
runs in its own worktree with setup/teardown commands driven by
.story_kit/project.toml config. Agents stream output via SSE and support
start/stop lifecycle with Pending/Running/Completed/Failed statuses.

Backend: config.rs (TOML parsing), worktree.rs (git worktree lifecycle),
refactored agents.rs (broadcast streaming), agents_sse.rs (SSE endpoint).
Frontend: AgentPanel.tsx with Run/Stop buttons and streaming output log.

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

server/src/config.rs

@@ -0,0 +1,145 @@
+use serde::Deserialize;
+use std::path::Path;
+
+#[derive(Debug, Clone, Deserialize)]
+pub struct ProjectConfig {
+    #[serde(default)]
+    pub component: Vec<ComponentConfig>,
+    pub agent: Option<AgentConfig>,
+}
+
+#[derive(Debug, Clone, Deserialize)]
+pub struct ComponentConfig {
+    pub name: String,
+    #[serde(default = "default_path")]
+    pub path: String,
+    #[serde(default)]
+    pub setup: Vec<String>,
+    #[serde(default)]
+    pub teardown: Vec<String>,
+}
+
+#[derive(Debug, Clone, Deserialize)]
+pub struct AgentConfig {
+    #[serde(default = "default_agent_command")]
+    pub command: String,
+    #[serde(default)]
+    pub args: Vec<String>,
+    #[serde(default = "default_agent_prompt")]
+    pub prompt: String,
+}
+
+fn default_path() -> String {
+    ".".to_string()
+}
+
+fn default_agent_command() -> String {
+    "claude".to_string()
+}
+
+fn default_agent_prompt() -> String {
+    "Read .story_kit/README.md, then pick up story {{story_id}}".to_string()
+}
+
+impl Default for ProjectConfig {
+    fn default() -> Self {
+        Self {
+            component: Vec::new(),
+            agent: Some(AgentConfig {
+                command: default_agent_command(),
+                args: vec![],
+                prompt: default_agent_prompt(),
+            }),
+        }
+    }
+}
+
+impl ProjectConfig {
+    /// Load from `.story_kit/project.toml` relative to the given root.
+    /// Falls back to sensible defaults if the file doesn't exist.
+    pub fn load(project_root: &Path) -> Result<Self, String> {
+        let config_path = project_root.join(".story_kit/project.toml");
+        if !config_path.exists() {
+            return Ok(Self::default());
+        }
+        let content =
+            std::fs::read_to_string(&config_path).map_err(|e| format!("Read config: {e}"))?;
+        toml::from_str(&content).map_err(|e| format!("Parse config: {e}"))
+    }
+
+    /// Render template variables in agent args and prompt.
+    pub fn render_agent_args(
+        &self,
+        worktree_path: &str,
+        story_id: &str,
+    ) -> Option<(String, Vec<String>, String)> {
+        let agent = self.agent.as_ref()?;
+        let render = |s: &str| {
+            s.replace("{{worktree_path}}", worktree_path)
+                .replace("{{story_id}}", story_id)
+        };
+        let command = render(&agent.command);
+        let args: Vec<String> = agent.args.iter().map(|a| render(a)).collect();
+        let prompt = render(&agent.prompt);
+        Some((command, args, prompt))
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+    use std::fs;
+
+    #[test]
+    fn default_config_when_missing() {
+        let tmp = tempfile::tempdir().unwrap();
+        let config = ProjectConfig::load(tmp.path()).unwrap();
+        assert!(config.agent.is_some());
+        assert!(config.component.is_empty());
+    }
+
+    #[test]
+    fn parse_project_toml() {
+        let tmp = tempfile::tempdir().unwrap();
+        let sk = tmp.path().join(".story_kit");
+        fs::create_dir_all(&sk).unwrap();
+        fs::write(
+            sk.join("project.toml"),
+            r#"
+[[component]]
+name = "server"
+path = "."
+setup = ["cargo check"]
+teardown = []
+
+[[component]]
+name = "frontend"
+path = "frontend"
+setup = ["pnpm install"]
+
+[agent]
+command = "claude"
+args = ["--print", "--directory", "{{worktree_path}}"]
+prompt = "Pick up story {{story_id}}"
+"#,
+        )
+        .unwrap();
+
+        let config = ProjectConfig::load(tmp.path()).unwrap();
+        assert_eq!(config.component.len(), 2);
+        assert_eq!(config.component[0].name, "server");
+        assert_eq!(config.component[1].setup, vec!["pnpm install"]);
+
+        let agent = config.agent.unwrap();
+        assert_eq!(agent.command, "claude");
+    }
+
+    #[test]
+    fn render_template_vars() {
+        let config = ProjectConfig::default();
+        let (cmd, args, prompt) = config.render_agent_args("/tmp/wt", "42_foo").unwrap();
+        assert_eq!(cmd, "claude");
+        assert!(args.is_empty());
+        assert!(prompt.contains("42_foo"));
+    }
+}