Breakdown: Four Parts, One Interlocking Whole

The four ABCD documents each deep-dive into a subsystem of the core engine. Their dependency relationships are as follows:

User input
    ↓
A. Core Conversation Engine
   QueryEngine (outer) → Agent Loop (inner) → tool calls
         ↓                                      ↓
B. Permissions & Security          C. Tool System
   Decides whether each              Implements the concrete
   operation is allowed              logic for each tool
         ↓                                      ↓
         └──────────────────────────────────────┘
                          ↓
                  D. Product Growth Design
                  Records user behavior, controls feature rollout, tracks costs

Recommended reading order: A → B → C → D. A is the backbone; B and C are the two subsystems A calls into; D is the cross-cutting data layer that spans all modules.