ADR-011: Faceted tag taxonomy; ADR-012: reuse SecondBrain vault

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).

docs/memory-system/03-architecture-decisions.md

@@ -47,7 +47,8 @@ Alternatives rejected · Status. Newest decisions extend the log; supersede rath
   Pure-prefix path filtering via memsearch `source_prefix` (would force directories back in).
 - **Trade-off accepted**: Tags give the *human/Obsidian* free filtering, but the *AI* gets
   nothing for free from tags — we must materialize them into a queryable index (see ADR-004).
-- **Status**: Accepted.
+- **Status**: **Refined by ADR-011** (type/ and project/ namespaces added; hierarchy-vs-facets
+  clarified). Core decision — flat vault, namespaced tags — stands.
 
 ## ADR-004 — SQLite + Sequel (Ruby) tag index as the knowledge-layer cache
 
@@ -176,6 +177,85 @@ Alternatives rejected · Status. Newest decisions extend the log; supersede rath
   suffices).
 - **Status**: Accepted (to be built — see 04-build-plan.md and 06-graphify-evaluation.md).
 
+## ADR-011 — Faceted tag taxonomy: six independent namespaces (refines ADR-003)
+
+_Date: 2026-06-04_
+
+- **Context**: ADR-003 introduced five namespaces (`tool/`, `client/`, `domain/`,
+  `convention/`, `scope/`). During vault-reuse assessment (ADR-012) it became clear that (1)
+  the existing SecondBrain vault uses a de-facto first-tag convention for note kind
+  (research/plan/log/adr/howto) that should be made explicit and machine-queryable, and (2)
+  for a freelancer working many projects per client, project identity deserves a first-class
+  namespace rather than being implied by `client/` or `domain/`.
+- **Decision**: Knowledge-vault notes are classified by **six independent, flat tag facets**
+  that sit side-by-side, never nested into one another:
+  - `type/` — note kind: `research`, `howto`, `adr`, `hub`, `plan`, `log`, `clip`, etc.
+  - `client/` — which client
+  - `project/` — which project (first-class; a freelancer's projects are the primary unit of
+    work)
+  - `domain/` — knowledge domain / topic area
+  - `tool/` — tool-specific knowledge
+  - `convention/` — conventions
+  - …plus `scope/global` or `scope/project` (retained from ADR-003)
+
+  Hierarchy and relationships are expressed via **hub notes** (`type/hub`), **wikilinks**, and
+  **Graphify knowledge-graph edges** — NOT via nested tag paths.
+
+  By convention `type/` is listed **first** in frontmatter, preserving the SecondBrain vault's
+  existing type-first ordering habit and making the note kind immediately visible.
+- **Rationale**: The vault is flat — hierarchy is not expressed through folder paths or tag
+  nesting. The user's reality is many-to-many (many projects per client, knowledge domains
+  spanning clients), which a single-parent tree models badly and forces false hierarchy. A
+  project hub note links out to both its `client/` and relevant `domain/` tags rather than
+  being buried under either. Per-type `_templates` will be provided for **core types only**
+  (research, howto, adr, hub); the long tail stays freeform until a pattern earns a template.
+  Consistent per-type structure also improves Graphify's local-SLM extraction reliability.
+- **Alternatives rejected**: Hierarchical nesting in the style of John Conneely's
+  `domain/{product}/{project}.md` folder structure (from the youngleaders.tech article "How I
+  finally sorted my Claude Code memory" — **secondary/interview-grade source, not verified
+  against primary implementation**). Rejected because: (1) the vault is flat — hierarchy is
+  not expressed through folder paths; (2) the user's many-to-many reality maps badly onto a
+  single-parent folder tree and forces false hierarchy; (3) nesting one facet through another
+  (e.g. `domain/client/project`) creates Law-of-Demeter-style traversal coupling. Conneely's
+  structure was the inspiration but diverges here on hierarchy-vs-facets. Faceted parallel tags
+  are the flat-vault analogue of what the Graphify graph already does with edges, so they
+  compose naturally with the chosen knowledge layer.
+- **Status**: Accepted (supersedes the namespace list in ADR-003; core flat-vault +
+  namespaced-tags decision stands).
+
+## ADR-012 — Reuse the existing SecondBrain vault as the knowledge vault
+
+_Date: 2026-06-04_
+
+- **Context**: The design called for a flat markdown vault as the semantic knowledge layer
+  (ADR-003/008/010). The question was whether to stand up a new `~/brain` vault from scratch
+  or adopt the existing `~/Documents/SecondBrain` vault.
+- **Decision**: **Adopt `~/Documents/SecondBrain`** as the knowledge vault rather than
+  creating a new vault.
+- **Rationale**: Assessment found the SecondBrain vault is already flat (all notes at root,
+  only a `_templates/` exception — exactly what the design permits), already articulates the
+  correct "durable knowledge, not working memory" role in its `CLAUDE.md` and
+  `vault-conventions.md`, and contains ~20 real notes. It also includes two patterns that
+  **improve on the current design** and should be adopted:
+  1. `vault-conventions.md`'s "act without being asked" section specifying *when* the AI
+     should proactively query the vault — a behavioral spec the cc-os docs lacked.
+  2. Project-config hub notes with a tag-inference table (auto-tag by path pattern) that
+     operationalizes *how* to tag a note from a given project.
+- **Adaptations required (migration cost)**:
+  - Add `summary:` frontmatter to existing notes.
+  - Migrate flat unnamespaced tags to the six-facet namespaced form (per ADR-011).
+  - Add `scope/global` or `scope/project` to each note.
+  - Initialize git in the vault (no `.git` exists yet — required by ADR-008's sync strategy).
+  - Replace the vault's `~/.claude/scripts/vault_search.rb` reference (script does not exist)
+    with `graphify query` (ADR-010).
+  These are mechanical schema migrations, not structural rework.
+- **Alternatives rejected**: Starting fresh with a new `~/brain` vault. Rejected because the
+  hardest design decision — flat structure, durable-knowledge-only role, governance philosophy
+  — is already made and practiced in SecondBrain. The improved behavioral patterns
+  (proactive-query spec, tag-inference table) and the existing notes are worth preserving; the
+  remaining work is mechanical migration.
+- **Status**: Accepted.
+
 ## Rejected tools (summary)
 
 | Tool | Why rejected for our use |