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/021-edge-identity-uses-consumer-keys.md

@@ -0,0 +1,23 @@
+# ADR-021: Edge identity uses consumer-defined keys
+
+## Status
+
+Accepted
+
+## Context
+
+Edges need unique identity within a graph. Auto-generated UUIDs vs consumer-defined keys affect how edges are referenced and updated.
+
+## Decision
+
+Edges use `(graphId, key)` as their unique identity, where `key` is consumer-defined. This matches the metagraph model where consumers control identifiers. For anonymous edges (common in simple graphs), `key` can be auto-generated.
+
+## Consequences
+
+- Consumers can reference edges by meaningful keys (e.g., `"call-001→call-002"`)
+- Updates use the consumer's key, not a generated UUID
+- Anonymous edges can have null or auto-generated keys
+
+## References
+
+- [sqlite-host.md](../sqlite-host.md)