docs: index missing reference guides in adk-agent-builder SKILL.md

Adds import-paths, advanced-patterns, session-and-state, and callbacks-and-plugins references to the main index file.

Co-authored-by: Shangjie Chen <deanchen@google.com>
PiperOrigin-RevId: 940676946

Author: DeanChensj

.agents/skills/adk-agent-builder/SKILL.md

@@ -5,27 +5,37 @@ description: Central hub for building, testing, and iterating on ADK agents. Tri
 
 # ADK Agent Builder
 
-This file serves as a directory of specialized reference guides for developing agents with ADK. To avoid context pollution, read only the relevant reference file based on your current task.
+This file serves as a directory of specialized reference guides for developing
+agents with ADK. To avoid context pollution, read only the relevant reference
+file based on your current task.
 
 ## Core Concepts Directory
 
 Refer to these files for foundational knowledge:
+
 - **Getting Started & Basic Agents**: [getting-started.md](references/getting-started.md)
   - Environment setup, API key configuration, and minimal agent definitions.
 - **Tool Catalog**: [tool-catalog.md](references/tool-catalog.md)
   - How to bind function tools, MCP tools, OpenAPI specs, and Google API tools.
 - **Agent Modes (Task / Single-Turn)**: [task-mode.md](references/task-mode.md)
   - Multi-turn structured delegation and autonomous single-turn execution patterns.
+-   **Import Paths**: [import-paths.md](references/import-paths.md)
+    -   Canonical and verbose import paths for core components, tools, and
+        events.
 
 ## Workflow & Graph Orchestration
 
 Refer to these files when building complex graphs:
+
 - **Function Nodes**: [function-nodes.md](references/function-nodes.md)
   - How to use functions as nodes, type resolution, and generators.
 - **Routing & Conditions**: [routing-and-conditions.md](references/routing-and-conditions.md)
   - Edge patterns, dict-based routing, self-loops, and conditional execution.
 - **LLM Agent Nodes**: [llm-agent-nodes.md](references/llm-agent-nodes.md)
   - How to use LLM agents as workflow nodes, task wrappers, and handling output schemas.
+-   **Advanced Patterns**:
+    [advanced-patterns.md](references/advanced-patterns.md)
+    -   Nested workflows, custom node types, and graph validation rules.
 
 ## Advanced Orchestration Patterns
 
@@ -40,6 +50,14 @@ Refer to these files when building complex graphs:
 
 - **State & Events**: [state-and-events.md](references/state-and-events.md)
   - Using context API, sharing global state, and yield event structures.
+-   **Session & Memory**:
+    [session-and-state.md](references/session-and-state.md)
+    -   Session state mutation, scope conventions, and database session
+        services.
+-   **Callbacks & Plugins**:
+    [callbacks-and-plugins.md](references/callbacks-and-plugins.md)
+    -   Implementing callbacks, plugin manager integration, and override
+        behavior.
 - **Multi-Agent Systems**: [multi-agent.md](references/multi-agent.md)
   - Hierarchical execution (e.g., `SequentialAgent`, `LoopAgent`, `ParallelAgent`).
 - **Testing Strategies**: [testing.md](references/testing.md)