tasks(arch): adopt taskgraph for architecture work — blocker tasks for deferred OQs, level mapping, status fixes

Establishes the tasks/architecture/ blocker-task half of the Safe Exit
protocol. Each deferred OQ now has an external-trigger tracker task
representing its unblocking condition, and the OQ's Blocked on field names
the tracker task ID — closing the gap where the deferral field existed but
the task-graph half was unenforced.

Changes:
- Add 6 external-trigger tracker tasks for the deferred OQs (OQ-09, 10, 32,
  41, 44, 46) under tasks/architecture/, tagged [external-trigger, deferred-oq]
- Add structured Blocked on field to OQ-09 and OQ-10 (previously used legacy
  'deferred' status with the reason in the Resolution prose); index now
  surfaces all 6 deferred OQs with concrete conditions, no placeholders
- Document the architecture-task level mapping (level: implementation for
  ADRs/specs, decomposition for spec-to-ADR breakdown, planning for backlog
  seeding, research/review direct fits) and the deferred-OQ blocker-task
  pattern in docs/sdd_process.md
- Mark architecture/safe-exit-blocker-task-mechanism and
  architecture/oq-09-10-blocking-conditions completed (implemented by this pass)
- Fix 4 pre-existing ADR-050 implementation tasks: status: done → completed
  (taskgraph enum is 'completed', not 'done') — taskgraph validate now passes
  for all 100 tasks

docs/architecture/open-questions.md

@@ -160,13 +160,13 @@ filtering the tables above.
 
 ### OQ-09: WASM Target Boundaries
 
-- **Blocked on**: _(no explicit blocking condition recorded — see full file)_
+- **Blocked on**: A concrete server-side WASM use case, or a deliberate confirmation that WASM stays a client-side design constraint. Tracked as `architecture/oq-09-wasm-server-use-case` in `tasks/architecture/`.
 - **Priority**: low
 - **Full file**: [OQ-09](questions/009-wasm-target-boundaries.md)
 
 ### OQ-10: Git Adapter Scope — Smart Protocol Only or Full Server?
 
-- **Blocked on**: _(no explicit blocking condition recorded — see full file)_
+- **Blocked on**: Speccing the alknet-git crate — resolve this when that crate is specified, not deferred past it. Tracked as `architecture/oq-10-git-adapter-spec` in `tasks/architecture/`.
 - **Priority**: low
 - **Full file**: [OQ-10](questions/010-git-adapter-scope-smart-protocol-only-or-full-server.md)