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/047-stdin-closure-canonical-signal.md

@@ -0,0 +1,21 @@
+# OQ-47: Stdin Closure Canonical Signal
+
+- **Origin**: `docs/research/alknet-docker/poc-summary.md` §"Open
+  Unknowns" #4 (stdin closure semantics for raw carriage);
+  [crates/tty/tty-wire.md](crates/tty/tty-wire.md) §"Stdin Closure".
+- **Status**: resolved
+- **Door type**: Two-way
+- **Priority**: low
+- **Resolution**: Either a zero-length stdin chunk (stream_type 0,
+  length 0 — the docker POC's sentinel) or a `{"type":"eof"}`
+  control chunk (stream_type 3 — the tty POC's explicit signal) closes
+  the client's stdin. Both are accepted by the adapter; the spec
+  recommends `eof` for explicitness (it's a control message, not a
+  data-length hack). The adapter handles both identically: signal EOF
+  to the backend's stdin (`ChildStdin::drop` / PTY writer close) and
+  keep pumping stdout until the exit resolves — the client may still
+  want to receive remaining output + the exit code. A third path
+  (client closes the write half of the bidi stream) is also accepted
+  and handled the same way. See ADR-052 and `tty-wire.md`.
+- **Cross-references**: ADR-052, [tty-wire.md](crates/tty/tty-wire.md),
+  [tty-adapter.md](crates/tty/tty-adapter.md)