fix: handle whitespace-variant moment separators

[jgordijn]

Summary

• make separator-based fragment splitting tolerant to internal whitespace variation like ~ ~ ~
• unwrap a single top-level container before splitting so wrapped fragment feeds still parse correctly
• clear fragment parsing state and existing fragment entries when fragment settings change so refetch rebuilds them cleanly
• align the main OpenSpec content-fetching spec with the whitespace-normalized separator behavior

Testing

• go test ./cmd/knowledgehub ./internal/engine/... -count=1

Notes

• This fixes the Ai Shepherd /moments feed case where the page documents ~ ~ ~ but the feed can emit ~ ~ ~.