feat: configurable fragment separator mode

[jgordijn]

What

Add per-resource fragment splitting configuration so fragment feeds can use either:

• Auto (default): existing heuristic + AI paragraph-based splitting
• Separated: split by an explicit text separator (e.g. ~~~ for aishepherd.nl/moments)

Changes

Backend

• SplitFragmentsBySeparator() in internal/engine/fragment.go — walks DOM children, groups elements between separator-matching elements
• Fetcher dispatch in internal/engine/fetcher.go — routes to separator or auto mode based on resource config
• DB migration in cmd/knowledgehub/collections.go — adds fragment_mode (select: auto/separated) and fragment_separator (text) fields
• Test collection schema in internal/testutil/testutil.go — includes new fields
• 6 new tests covering: basic split, not found, leading/trailing, consecutive separators, different separators, mixed elements

Frontend

• ResourceForm.svelte — split mode dropdown + separator text input, shown when Fragment feed is checked
• resources/+page.svelte — passes new props in edit flow, badge shows Fragment (~~~) for separated mode

OpenSpec

• Archived change with proposal, design, specs, and tasks
• Synced delta specs to content-fetching and resource-management main specs

Testing

All existing tests pass. 6 new SplitFragmentsBySeparator tests pass. Frontend builds cleanly.