docs: add architecture notes and development principles

doc/architecture.md

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+# Code Architecture
+
+- plyngent
+    - typedef: Shared type aliases.
+    - lmproto: Protocols for interacting with LLM service.
+        - (common)
+            - model: Messages models of specified providers.
+            - config: Configurations for specified providers.
+            - client: Clients for accessing existent specified services.
+            - server: Servers for accepting other clients.
+        - openai
+        - openai_compatible
+        - anthropic
+        - ollama
+        - deepseek
+            - openai_compat
+            - anthropic
+    - utils: Common utilities for code architecture.
+        - components: Utilities for class composition.
+    - memory: Storage controlling for sessions and messages.
+    - router: Multi-source capability routing and interception.
+        - interceptors: Capability of stealing a few arguments for partial re-routing.
+        - ...
+    - config: Plyngent configuration center.
+    - agent: Agent capabilities and controlling.
+    - tools: Client code of tool calls.
+    - cli: CLI for chat/agent application.
+    - web: Web service for chat/agent application.
+