Story 33: Copy-paste diff commands for agent worktrees

- Add base_branch detection to WorktreeInfo (from project root HEAD)
- Expose base_branch in AgentInfo API response
- Add {{base_branch}} template variable to agent config rendering
- Show git difftool command with copy-to-clipboard in AgentPanel UI
- Add diff command instruction to coder agent prompts
- Add AgentPanel tests for diff command rendering and clipboard

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

server/src/agents.rs

@@ -76,6 +76,7 @@ pub struct AgentInfo {
     pub status: AgentStatus,
     pub session_id: Option<String>,
     pub worktree_path: Option<String>,
+    pub base_branch: Option<String>,
 }
 
 struct StoryAgent {
@@ -182,7 +183,7 @@ impl AgentPool {
         // Spawn the agent process
         let wt_path_str = wt_info.path.to_string_lossy().to_string();
         let (command, args, prompt) =
-            config.render_agent_args(&wt_path_str, story_id, Some(&resolved_name))?;
+            config.render_agent_args(&wt_path_str, story_id, Some(&resolved_name), Some(&wt_info.base_branch))?;
 
         let sid = story_id.to_string();
         let aname = resolved_name.clone();
@@ -247,6 +248,7 @@ impl AgentPool {
             status: AgentStatus::Running,
             session_id: None,
             worktree_path: Some(wt_path_str),
+            base_branch: Some(wt_info.base_branch.clone()),
         })
     }
 
@@ -321,6 +323,10 @@ impl AgentPool {
                         .worktree_info
                         .as_ref()
                         .map(|wt| wt.path.to_string_lossy().to_string()),
+                    base_branch: agent
+                        .worktree_info
+                        .as_ref()
+                        .map(|wt| wt.base_branch.clone()),
                 }
             })
             .collect())

server/src/config.rs

@@ -175,6 +175,7 @@ impl ProjectConfig {
         worktree_path: &str,
         story_id: &str,
         agent_name: Option<&str>,
+        base_branch: Option<&str>,
     ) -> Result<(String, Vec<String>, String), String> {
         let agent = match agent_name {
             Some(name) => self
@@ -185,9 +186,11 @@ impl ProjectConfig {
                 .ok_or_else(|| "No agents configured".to_string())?,
         };
 
+        let bb = base_branch.unwrap_or("master");
         let render = |s: &str| {
             s.replace("{{worktree_path}}", worktree_path)
                 .replace("{{story_id}}", story_id)
+                .replace("{{base_branch}}", bb)
         };
 
         let command = render(&agent.command);
@@ -378,7 +381,7 @@ max_turns = 0
     fn render_agent_args_default() {
         let config = ProjectConfig::default();
         let (cmd, args, prompt) = config
-            .render_agent_args("/tmp/wt", "42_foo", None)
+            .render_agent_args("/tmp/wt", "42_foo", None, None)
             .unwrap();
         assert_eq!(cmd, "claude");
         assert!(args.is_empty());
@@ -404,7 +407,7 @@ max_turns = 30
 
         let config = ProjectConfig::parse(toml_str).unwrap();
         let (cmd, args, prompt) = config
-            .render_agent_args("/tmp/wt", "42_foo", Some("supervisor"))
+            .render_agent_args("/tmp/wt", "42_foo", Some("supervisor"), Some("master"))
             .unwrap();
         assert_eq!(cmd, "claude");
         assert!(args.contains(&"--model".to_string()));
@@ -422,7 +425,7 @@ max_turns = 30
 
         // Render for coder
         let (_, coder_args, _) = config
-            .render_agent_args("/tmp/wt", "42_foo", Some("coder"))
+            .render_agent_args("/tmp/wt", "42_foo", Some("coder"), Some("master"))
             .unwrap();
         assert!(coder_args.contains(&"sonnet".to_string()));
         assert!(coder_args.contains(&"30".to_string()));
@@ -433,7 +436,7 @@ max_turns = 30
     #[test]
     fn render_agent_args_not_found() {
         let config = ProjectConfig::default();
-        let result = config.render_agent_args("/tmp/wt", "42_foo", Some("nonexistent"));
+        let result = config.render_agent_args("/tmp/wt", "42_foo", Some("nonexistent"), None);
         assert!(result.is_err());
         assert!(result.unwrap_err().contains("No agent named 'nonexistent'"));
     }

server/src/worktree.rs

@@ -7,6 +7,7 @@ use std::process::Command;
 pub struct WorktreeInfo {
     pub path: PathBuf,
     pub branch: String,
+    pub base_branch: String,
 }
 
 /// Worktree path as a sibling of the project root: `{project_root}-story-{id}`.
@@ -24,6 +25,23 @@ fn branch_name(story_id: &str) -> String {
     format!("feature/story-{story_id}")
 }
 
+/// Detect the current branch of the project root (the base branch worktrees fork from).
+fn detect_base_branch(project_root: &Path) -> String {
+    Command::new("git")
+        .args(["rev-parse", "--abbrev-ref", "HEAD"])
+        .current_dir(project_root)
+        .output()
+        .ok()
+        .and_then(|o| {
+            if o.status.success() {
+                Some(String::from_utf8_lossy(&o.stdout).trim().to_string())
+            } else {
+                None
+            }
+        })
+        .unwrap_or_else(|| "master".to_string())
+}
+
 /// Create a git worktree for the given story.
 ///
 /// - Creates the worktree at `{project_root}-story-{story_id}` (sibling directory)
@@ -37,6 +55,7 @@ pub async fn create_worktree(
 ) -> Result<WorktreeInfo, String> {
     let wt_path = worktree_path(project_root, story_id);
     let branch = branch_name(story_id);
+    let base_branch = detect_base_branch(project_root);
     let root = project_root.to_path_buf();
 
     // Already exists — reuse
@@ -45,6 +64,7 @@ pub async fn create_worktree(
         return Ok(WorktreeInfo {
             path: wt_path,
             branch,
+            base_branch,
         });
     }
 
@@ -60,6 +80,7 @@ pub async fn create_worktree(
     Ok(WorktreeInfo {
         path: wt_path,
         branch,
+        base_branch,
     })
 }