Add timeout and curl fallback for feed fetching

[lichory]

Summary

• add a timeout wrapper around native fetch calls in scripts/prepare-digest.js
• fall back to curl when fetch times out or throws
• keep the JSON output contract unchanged while improving reliability

Why

In some environments, the GitHub raw feeds are reachable, but Node's native fetch() can hang or fail with fetch failed. That causes prepare-digest.js to error even though the upstream feed is actually available.

This patch makes the skill more robust for gateway, proxy, TLS-sensitive, and scheduled automation environments.

Notes

• verified locally against the existing feed URLs
• preserves existing output shape and behavior on successful fetch() paths