Lock two design decisions: (1) six independent namespaced tag facets
(type/client/project/domain/tool/convention/) plus scope/, with
hierarchy expressed via hub notes and Graphify edges, not nested paths;
(2) adopt the existing ~/Documents/SecondBrain vault as the knowledge
layer rather than creating a new one — it is already flat, governs to
the correct semantics, and contains useful patterns (proactive-query
spec, tag-inference table).
See ADR-011 and ADR-012 for rationale, rejected alternatives, and
migration cost (mechanical schema updates on existing ~20 notes).
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