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/025-password-based-encryption-pbkdf2.md

@@ -0,0 +1,26 @@
+# ADR-025: Password-based encryption via PBKDF2
+
+## Status
+
+Accepted
+
+## Context
+
+Encryption can use PBKDF2 key derivation from a password string or direct AES-GCM with raw key bytes. The hub uses password-based encryption.
+
+## Decision
+
+Use the PBKDF2 pattern for consistency with the hub. The "password" in practice is a base64-encoded 32-byte random key from `generateEncryptionKey()`. PBKDF2 adds security per-encryption (each encryption gets a unique salt), but adds ~100ms latency per operation.
+
+If performance becomes an issue, add an `encryptRaw()` function that skips PBKDF2 for raw key inputs.
+
+## Consequences
+
+- Consistent with hub's crypto implementation
+- ~100ms per encrypt/decrypt due to PBKDF2 iterations
+- v1 key version uses 100,000 PBKDF2 iterations
+- Web Crypto API only — no external crypto dependencies
+
+## References
+
+- [encrypted-data.md](../encrypted-data.md)