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>

frontend/src/api/agents.ts

@@ -6,6 +6,7 @@ export interface AgentInfo {
 	status: AgentStatusValue;
 	session_id: string | null;
 	worktree_path: string | null;
+	base_branch: string | null;
 }
 
 export interface AgentEvent {