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/004-injectable-clients-no-side-effects.md

@@ -0,0 +1,24 @@
+# ADR-004: Injectable clients, no module-level side effects
+
+## Status
+
+Accepted
+
+## Context
+
+Module-level side effects (auto-connections, env-based configuration) make testing difficult and conflict with containerized deployment patterns. The hub needs to create database instances with different configurations.
+
+## Decision
+
+`createSqliteDatabase(client)` receives a pre-created client. Module-level side effects are forbidden. The factory enables in-memory testing (`createClient({ url: ":memory:" })`) and custom client configuration.
+
+## Consequences
+
+- No global database state — each consumer explicitly creates and injects
+- Testing with in-memory databases is straightforward
+- Containerized deployment patterns are supported
+- Consumer controls client lifecycle (connections, cleanup)
+
+## References
+
+- [sqlite-host.md](../sqlite-host.md)