Add shared document renderer dependency

Why: Task 10 requires Backend to consume the reviewed canonical renderer and its embedded organization-default candidate without copying assets.
Evidence: Fresh GOMODCACHE resolution loaded agent-document-renderer/profiles and passed its package tests at d449b9f779f2f968f8ac556b89940a6c9aaf3fdc.
Impact: Adds one immutable Renderer gitlink plus the local Go module require/replace; no production path changes yet.
Refs: Unified Document Renderer Task 10; Renderer d449b9f779f2f968f8ac556b89940a6c9aaf3fdc

.gitmodules

@@ -19,3 +19,6 @@
 [submodule "deps/agent-command"]
 	path = deps/agent-command
 	url = ssh://git@git-ssh.u1680832.nyat.app:31411/weichen/Agent-Command.git
+[submodule "deps/agent-document-renderer"]
+	path = deps/agent-document-renderer
+	url = ssh://git@git-ssh.u1680832.nyat.app:31411/weichen/Agent-Document-Renderer.git

go.mod

@@ -4,6 +4,7 @@ go 1.27
 
 require (
 	agent-command v0.0.0
+	agent-document-renderer v0.0.0
 	agent-intent v0.0.0
 	agent-native v0.0.0
 	agent-squad v0.0.0
@@ -52,3 +53,5 @@ replace error-code-system => ./deps/error-code-system
 replace logging-system => ./deps/logging-system
 
 replace agent-command => ./deps/agent-command
+
+replace agent-document-renderer => ./deps/agent-document-renderer