docs(agent-note): flip the regrouping note to implemented

Move the triplet to implemented/architecture/, rewrite Proposal into a
present-tense Decision, fold Acceptance criteria and Risks into
Consequences (recording what pins the result), retitle the migration
section to what the move actually touched, and repoint the seven
inbound FIXME/README links.

packages/subagent/subagent-codex/tsconfig.json

@@ -24,7 +24,7 @@
       "path": "../../llm/llm"
     },
     {
-      "path": "../../sdk/sdk-protocol"
+      "path": "../../scaffold/protocol"
     },
     {
       "path": "../../core/session"

packages/subagent/subagent/tsconfig.json

@@ -30,10 +30,10 @@
       "path": "../../session/session-persistence"
     },
     {
-      "path": "../../session-projection/session-projection"
+      "path": "../../session/session-projection"
     },
     {
-      "path": "../../session-projection/session-projection-cache"
+      "path": "../../session/session-projection-cache"
     },
     {
       "path": "../../tasks/tasks"