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/032-multi-hop-federation.md

@@ -0,0 +1,19 @@
+# OQ-32: Multi-Hop Federation
+
+- **Origin**: [ADR-029](decisions/029-peer-graph-routing-model.md) §3.7, `docs/research/alknet-call-peer-routing/findings.md` §3.7
+- **Status**: deferred(scope)
+- **Door type**: One-way (federation model), two-way (mechanism)
+- **Priority**: low
+- **Blocked on**: A concrete use case for multi-hop federation. The one-hop model covers all current use cases (head→worker, runner→hub).
+- **Resolution**: The model is **one-hop** — worker A does not transitively
+  see worker B's ops through the head unless the head explicitly re-exports
+  them. The peer-keyed overlay model extends to multi-hop without redesign
+  (a chain of `PeerRef::Specific` routing decisions), but path-finding
+  (which peer reaches which op transitively) is where a graph library
+  (petgraph) would pay off. For one-hop (shallow), a nested
+  `HashMap<PeerId, HashMap<String, ...>>` suffices. Multi-hop federation is
+  a feature extension — the one-hop model is the architectural commitment;
+  extending to multi-hop doesn't break downstream crates. Whether multi-hop
+  becomes a real use case is a future decision; the peer-keyed model does
+  not foreclose it.
+- **Cross-references**: ADR-029, [client-and-adapters.md](crates/call/client-and-adapters.md)