fix(atlas): capture stderr from git commands to prevent help text leak

When git commands fail (e.g., not in a repo, invalid HEAD), git outputs
help text to stderr. Without explicit stdio option, execSync inherits
the parent's stdio causing help text to appear on terminal during
delegate_task execution.

Add stdio: ['pipe', 'pipe', 'pipe'] to capture stderr instead of
letting it leak to terminal.

src/hooks/atlas/index.ts

@@ -274,6 +274,7 @@ function getGitDiffStats(directory: string): GitFileStat[] {
       cwd: directory,
       encoding: "utf-8",
       timeout: 5000,
+      stdio: ["pipe", "pipe", "pipe"],
     }).trim()
 
     if (!output) return []
@@ -282,6 +283,7 @@ function getGitDiffStats(directory: string): GitFileStat[] {
       cwd: directory,
       encoding: "utf-8",
       timeout: 5000,
+      stdio: ["pipe", "pipe", "pipe"],
     }).trim()
 
     const statusMap = new Map<string, "modified" | "added" | "deleted">()