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/029-version-as-breaking-change-signal.md

@@ -0,0 +1,24 @@
+# ADR-029: Version as a coarse-grained breaking-change signal
+
+## Status
+
+Accepted
+
+## Context
+
+The `version` integer on `graph_types` could track every schema change or only breaking changes. Tracking every change is noisy; ignoring versioning creates risk.
+
+## Decision
+
+The `version` integer tracks **breaking** schema changes. Non-breaking changes (additive) do not require a version bump. The repository layer checks the version before processing and knows to run migration logic when it doesn't match. An even/odd scheme signals migration state: even = stable, odd = migration in progress.
+
+## Consequences
+
+- Most schema changes (additive) don't bump the version
+- Breaking changes bump the version and trigger migration
+- Odd versions signal incomplete migration for crash recovery
+- The consumer defines when a version bump occurs via a constant
+
+## References
+
+- [schema-evolution.md](../schema-evolution.md)