First of four canonical loaders. Reads canonical-tag-tree.xlsx by header
name, upserts each tag via TagService.upsertBySourceRef (never the
repository — layering rule), and resolves parent links by stripping the
last /segment of the canonical tag_path. Idempotent by source_ref.
Refs #669
Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>