feat(mcp): add a deep research tool that is off by default

.env.example

@@ -1273,6 +1273,12 @@ NEXT_PUBLIC_ENABLE_SOCKS5_PROXY=true
 # Full list: admin, combos, health, models, routing, budget, metrics, pricing, memory, skills
 # OMNIROUTE_MCP_SCOPES=admin,combos,health
 
+# Register the Perplexity Deep Research MCP tool. Unset or any value other
+# than 1 leaves it unregistered. A research run holds one connection for up to
+# 45 minutes, so it stays off unless an operator turns it on.
+# Used by: open-sse/mcp-server/tools/perplexityResearch.ts.
+# OMNIROUTE_PPLX_RESEARCH_ENABLED=0
+
 # Compress MCP tool descriptions before serializing the manifest.
 # Used by: open-sse/mcp-server/descriptionCompressor.ts — reduces token spend
 # for clients that read the full tool catalog.