chore: (skill): Add adk-unit-design skill definition

PiperOrigin-RevId: 937600184

Author: joefernandez

.agents/skills/adk-unit-design/SKILL.md

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+---
+name: adk-unit-design
+description: Creates or updates code unit design documents for source code documentation.
+---
+
+# ADK Code Unit Design
+
+This skill creates or updates a detailed software engineering design document for new or updated code file or specified code unit. The design document it generates is meant to explain the code to a developer who wants to modify or extend the code unit as part of the ADK development framework. Similar to a *unit test*, a *unit design* provides a generated software engineering design based on the *actual, implemented code* rather than any proposed code design or proposed software architecture.
+
+## Input
+
+- Code files containing new functionality
+- Names of new methods and classes (optional)
+- Code files for base classes or interfaces that the new functionality depends on (optional)
+- Code unit tests (optional)
+- Example code files (optional)
+
+## Analysis
+
+- Review specified code files for changes and named methods to determine:
+  - Purpose and intended use of the new or updated code units
+  - Any data flows handled by the new or updated code units
+  - Dependencies required by the new or updated code units
+  - Approaches for extending or customizing the code unit to add new capabilities
+  - Classes that depend on the new or updated code units
+  - Operational limitations of the new or updated code units
+
+## Output
+
+- Look for an existing design document in the `/docs/design/***` directory of this repository.
+  - If a design already exists, update the existing design incrementally and prioritize preserving the previous content as much as possible.
+  - If no design document exists, create a design file for the new code unit in the `/docs/design/***` directory of this repository, using the relative path of the code unit. For example, if the code unit is called `/topic/function/class.ext`, create a design document in the location `/docs/design/topic/function/class/index.md`.
+- Any links to local code files should be translated to URL links to the `google/adk-python` repository on GitHub. For example, if the local code unit path is `***/adk-python/topic/function/class.ext#L93`, the URL to the code file should be `https://github.com/google/adk-python/blob/main/topic/function/class.ext#L93`.
+
+### Design document structure and content
+
+Use the following structure and instructions to create the design document for the code unit:
+
+```
+# (name of code unit or code file) - Code Unit Design
+
+- 2-sentence summary of the code unit
+
+## Introduction
+
+- Paragraph(s) explaining:
+  - The purpose and application of the code unit, including intended use cases
+  - Developer problems solved by this code unit
+  - Agent capabilities enabled by this code unit
+
+## High-level architecture
+
+- Describe the software architecture of this code unit and how it fits into the larger ADK framework
+- Explain general execution flow of this code unit
+- Describe any data flows handled by the code unit including inputs and outputs
+- Explain any cross-class dependencies of the code unit, including upstream dependencies and downstream dependencies
+
+### Extension points
+
+- Describe how the code unit could be extended or customized to add new features or capabilities
+- Note specific parts of the code unit that are designed to be extended or customized, including:
+  - Abstract classes
+  - Interfaces
+  - Hooks
+  - Callbacks
+  - Configurable parameters
+  - Plugin architecture
+  - Other extension points
+
+### Extension constraints
+
+- Describe what parts of the code unit should not be modified, based on:
+  - architectural constraints
+  - implementation limitations
+  - cross-class dependencies
+  - other constraints
+
+## Limitations
+
+- Mention any limitations of the code unit, if known, such as:
+  - input constraints
+  - data structure constraints
+  - output constraints
+  - performance limitations
+  - memory limitations
+  - other limitations
+
+```