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.

tasks/architecture/open-questions-decompose.md

@@ -0,0 +1,54 @@
+---
+id: architecture/open-questions-decompose
+name: Decompose open-questions.md into per-OQ files under questions/
+status: completed
+depends_on: []
+scope: moderate
+risk: low
+impact: project
+level: architecture
+---
+
+## Description
+
+The single `docs/architecture/open-questions.md` file had grown to 1310 lines
+across 47 OQs — large enough to be unmanageable, with high size variance
+(OQ-42 at 220 lines next to OQ-06 at 8 lines). 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 + a cross-theme "Deferred / Blocked" section for safe-exit visibility).
+
+## What changed
+
+- Created `docs/architecture/questions/` with 47 files (`001-...md` through
+  `047-...md`), content moved verbatim from the old monolithic file (heading
+  rewritten from `### OQ-NN` to `# OQ-NN` for standalone files).
+- Rewrote `docs/architecture/open-questions.md` as the index:
+  - Status-values / door-type definitions preserved
+  - 15 theme sections, each with a table: `| OQ | Title | Status | Door | Pri |`
+  - A "Deferred / Blocked" cross-theme section surfacing the 6 deferred OQs
+    with their `Blocked on:` conditions inline (the safe-exit visibility
+    surface)
+- Updated `docs/architecture/README.md`: dropped the 65-line curated OQ summary
+  section (replaced by the index tables), kept the doc-table row pointing at
+  `open-questions.md`.
+- Original monolithic file preserved at `/tmp/opencode/open-questions.old.md`
+  for verification.
+
+## Verification
+
+- `python3 /tmp/opencode/verify_oqs.py` confirms all 47 per-OQ files match the
+  original content byte-for-byte (modulo the `### OQ` → `# OQ` heading rewrite).
+- All 62 inbound links to `open-questions.md` across ~30 files remain valid
+  (none used `#oq-NN` anchors — they all point at the bare filename, which is
+  unchanged).
+
+## Out of scope
+
+- Re-resolving or editing any OQ content — verbatim move only.
+- Fixing OQ-09/OQ-10's missing `Blocked on:` field — tracked as
+  `architecture/oq-09-10-blocking-conditions`.
+- Establishing the `tasks/architecture/` blocker-task convention — tracked as
+  `architecture/safe-exit-blocker-task-mechanism`.
+- Touching reviews/research docs that cite `open-questions.md` by line number
+  (those citations are already stale; reviews are historical artifacts).