Tag Hierarchy and Nested Tags — pyrite

Support hierarchical tags using `/` separator:

`#project/pyrite/backend`, `#project/pyrite/frontend`, `#source/interview`

Tag tree view in sidebar (collapsible hierarchy)

Filtering by parent tag includes all children

Autocomplete suggests existing tag paths

Backend: store flat but query with prefix matching (`tag LIKE 'project/pyrite/%'`)

Migration: existing flat tags continue to work

Obsidian power feature. Flat tags don't scale for research KBs with hundreds of tags — journalists tagging sources, topics, and projects need hierarchy.

Tag Hierarchy and Nested Tagsbacklog_item

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