feat: Add Graphviz diagram rendering in plan mode (#266)

* feat: add Graphviz rendering for plan mode

Support graphviz/dot/gv fenced blocks in the shared viewer and add a Graphviz sample diagram to PLAN_CONTENT for quick validation.

* fix: use imperative wheel listener with passive:false in GraphvizBlock

Author: flex-yj-kim

bun.lock

@@ -76,6 +76,26 @@ flowchart LR
     WS <--> WSS
 \`\`\`
 
+### Service Dependencies (Graphviz)
+
+\`\`\`graphviz
+digraph CollaborationStack {
+  rankdir=LR;
+  node [shape=box, style="rounded"];
+
+  Browser [label="Client Browser"];
+  API [label="WebSocket API"];
+  OT [label="OT Engine"];
+  Redis [label="Presence Cache"];
+  Postgres [label="PostgreSQL"];
+
+  Browser -> API;
+  API -> OT;
+  OT -> Redis;
+  OT -> Postgres;
+}
+\`\`\`
+
 ## Phase 1: Infrastructure
 
 ### WebSocket Server