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/017-abort-cascade-semantics-for-nested-calls.md

@@ -0,0 +1,8 @@
+# OQ-17: Abort Cascade Semantics for Nested Calls
+
+- **Origin**: [call-protocol.md](crates/call/call-protocol.md), [operation-registry.md](crates/call/operation-registry.md)
+- **Status**: resolved
+- **Door type**: One-way (protocol schema), two-way (mechanism)
+- **Priority**: high
+- **Resolution**: `call.aborted` cascades to all non-terminal descendants in the call tree. The CallAdapter walks the tree (indexed by `parent_request_id` in `PendingRequestMap`) and sends `call.aborted` for each descendant. Default policy is `abort-dependents` (abort everything downstream); `continue-running` is an opt-in for long-running work that should survive a parent's abort. Handlers clean up via Rust's async drop semantics (future dropped → `Drop` guards release resources). The cascade is protocol-level (server discovers descendants and propagates); the mechanism (parent-indexed map, cancellation tokens, or a separate graph) is a two-way door. See ADR-016.
+- **Cross-references**: ADR-012, ADR-015, ADR-016, [call-protocol.md](crates/call/call-protocol.md), [operation-registry.md](crates/call/operation-registry.md)