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())