docs: restructure architecture docs to flowgraph pattern

- Create decisions/ directory with 32 numbered ADRs (ADR-001 through ADR-032)
  extracted from inline DD/SD/ED/SE decision sections
- Create open-questions.md with 16 OQs organized by theme, cross-referenced
  to ADRs, with status tracking (resolved/open)
- Create README.md as architecture index with doc table, ADR table, and
  lifecycle status definitions (draft/reviewed/stable/deprecated)
- Replace inline decision sections in all spec docs with ADR reference tables
- Replace inline open questions with OQ references to centralized tracker
- Update frontmatter: metagraph-module.md, overview.md, sqlite-host.md → reviewed;
  schema-evolution.md and encrypted-data.md remain draft
- DD1-DD10 → ADR-009 through ADR-018
- D1-D8 → ADR-001 through ADR-008
- SD1-SD5 → ADR-019 through ADR-023 (SD5 folded into ADR-006/008)
- ED1-ED5 → ADR-023 through ADR-027
- SE1-SE5 → ADR-028 through ADR-032

docs/architecture/decisions/019-json-text-for-schema-columns.md

@@ -0,0 +1,24 @@
+# ADR-019: JSON text for schema columns in SQLite
+
+## Status
+
+Accepted
+
+## Context
+
+SQLite stores JSON as `text` with `{ mode: "json" }`. PostgreSQL uses native `jsonb`. The choice affects queryability and validation behavior.
+
+## Decision
+
+SQLite uses `text` with JSON mode for `schema`, `config`, `attributes`, `metadata`, and `allowedSourceTypes`/`allowedTargetTypes` columns. JSON validation relies on application-level TypeBox schemas, not database constraints. SQLite is for spokes (local, infrequent queries); PostgreSQL is for the hub (frequent, complex queries where `jsonb` queryability matters).
+
+## Consequences
+
+- SQLite cannot efficiently query inside JSON columns (no GIN indexes)
+- All JSON validation is application-level (`Value.Check`)
+- PostgreSQL gets queryability benefits from `jsonb` when implemented
+- The dual-host strategy is appropriate: SQLite for local infrequent access, PG for hub-level querying
+
+## References
+
+- [sqlite-host.md](../sqlite-host.md)