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

tasks/core/ownership-store-trait.md

@@ -1,7 +1,7 @@
 ---
 id: core/ownership-store-trait
 name: Add OwnershipProvider (sync read) + OwnershipStore (async write) traits and InMemoryOwnershipStore (ADR-050)
-status: done
+status: completed
 depends_on: []
 scope: moderate
 risk: low