discoverPiUserExtensions passed any *.ts/*.js directory entry as an
explicit --extension path, including macOS AppleDouble sidecars like
"._name.ts". Pi treats a parse failure on an explicit extension path as
fatal, so one stale sidecar in ~/.pi/agent/extensions killed every Pi
turn with an opaque "pi process closed stdout" error while interactive
pi kept working. Skip dot entries, matching pi's own loader.

Reported by @astarktc in #7211.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>