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/client/channel-manager.md

@@ -1,7 +1,7 @@
 ---
 id: client/channel-manager
 name: Implement ChannelManager — SSH session management, channel opens, reconnection
-status: done
+status: completed
 depends_on:
   - auth/client-auth-handler
   - transport/trait-and-types

tasks/client/connect-options.md

@@ -1,7 +1,7 @@
 ---
 id: client/connect-options
 name: Implement ConnectOptions struct and client session orchestration with graceful shutdown
-status: pending
+status: completed
 depends_on:
   - client/channel-manager
   - client/socks5-server

tasks/client/port-forwarding.md

@@ -1,7 +1,7 @@
 ---
 id: client/port-forwarding
 name: Implement port forwarding — local (-L) and remote (-R) forwards
-status: pending
+status: completed
 depends_on:
   - auth/client-auth-handler
   - transport/trait-and-types

tasks/client/socks5-server.md

@@ -1,7 +1,7 @@
 ---
 id: client/socks5-server
 name: Implement SOCKS5 server — local proxy that forwards through SSH channels
-status: pending
+status: completed
 depends_on:
   - auth/client-auth-handler
   - transport/trait-and-types