Add 10 new tasks under tasks/architecture/ for Phase 0a (ADR writing):
- 9 ADR tasks (026-034) with dependency-ordered structure
- 1 review checkpoint task before Phase 0b spec writing
ADR dependency graph (3 generations):
Gen 1 (parallel): 026, 029, 030, 031, 032, 034
Gen 2 (depends on 029): 027, 028
Gen 3 (depends on 027+028): 033
Gen 4: review checkpoint
Also mark all 34 prior implementation tasks as completed — they
were finished but still showing as pending in the taskgraph.
