docs(arch): decompose open-questions.md into per-OQ files under questions/

The monolithic open-questions.md (1310 lines, 47 OQs) was large enough to be
unmanageable, with high size variance (OQ-42 at 220 lines next to OQ-06 at 8).
Decomposed into one file per OQ under docs/architecture/questions/ (NNN-slug.md,
mirroring the ADR convention), with open-questions.md retained as the index:
theme-grouped tables plus a cross-theme Deferred/Blocked section that surfaces
the 6 deferred OQs with their Blocked-on conditions inline (the safe-exit
visibility surface). Per-OQ content moved verbatim; all 62 inbound links stay
valid (none used anchors). README's curated OQ summary dropped (now redundant
with the index tables).

Also seeds tasks/architecture/ with this task plus two follow-ups found during
the decompose: OQ-09/10 missing structured Blocked-on fields, and the
tasks/architecture/ blocker-task half of the Safe Exit protocol being
unenforced.

docs/architecture/questions/020-salt-kdf-and-encryption-key-derivation-method.md

@@ -0,0 +1,8 @@
+# OQ-20: Salt/KDF and Encryption Key Derivation Method
+
+- **Origin**: [encryption.md](crates/vault/encryption.md)
+- **Status**: resolved
+- **Door type**: One-way (key derivation method), two-way (salt field usage)
+- **Priority**: high
+- **Resolution**: The vault uses SLIP-0010 HD derivation from the BIP39 seed at path `m/74'/2'/0'/0'` to produce the AES-256-GCM encryption key — not PBKDF2. The `salt` field in `EncryptedData` is unused for key derivation (kept for wire-format compatibility with the TS predecessor). The TypeScript `@alkdev/storage` crypto module used PBKDF2 with a password + salt; data encrypted by that method (key_version=1) cannot be decrypted by the vault and must be migrated via one-time re-encryption to key_version=2. See ADR-020 for the full rationale and migration path.
+- **Cross-references**: ADR-020, [encryption.md](crates/vault/encryption.md)