tasks: decompose Phase 0a ADR foundation and mark prior tasks completed

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.

tasks/server/handler.md

@@ -1,7 +1,7 @@
 ---
 id: server/handler
 name: Implement ServerHandler — russh server handler with auth and channel dispatch
-status: pending
+status: completed
 depends_on:
   - auth/server-auth-handler
   - transport/trait-and-types