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/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,
     })
 }