Fix busy loop in `tart exec -i` after piped stdin reaches EOF (#1281)

Unregister the stdin readabilityHandler when availableData returns empty:
a closed pipe fd stays permanently readable, so Foundation re-invokes the
handler in a tight loop (fstat + zero-byte read) at 100% of one core for
the rest of the command's lifetime.

Fixes #1280

Co-authored-by: Claude Fable 5 <noreply@anthropic.com>

Sources/tart/Commands/Exec.swift

@@ -129,6 +129,11 @@ struct Exec: AsyncParsableCommand {
               let data = handle.availableData
 
               if data.isEmpty {
+                // EOF: unregister the handler, otherwise the fd stays permanently
+                // "readable" and Foundation re-invokes us in a tight loop, burning
+                // 100% of a core for the rest of the command's lifetime
+                handle.readabilityHandler = nil
+
                 continuation.finish()
               } else {
                 continuation.yield(data)