chore(skills): Add unit guide skill

Merge #6075

Creating Skill for generating code unit guides. See skill description for details

### Testing Plan

Not applicable

### Checklist

- [x] I have read the [CONTRIBUTING.md](https://github.com/google/adk-python/blob/main/CONTRIBUTING.md) document.
- [x] I have performed a self-review of my own code.
- [x] I have commented my code, particularly in hard-to-understand areas.
- [n/a] I have added tests that prove my fix is effective or that my feature works.
- [n/a] New and existing unit tests pass locally with my changes.
- [x] I have manually tested my changes end-to-end.
- [x] Any dependent changes have been merged and published in downstream modules.

COPYBARA_INTEGRATE_REVIEW=#6075 from google:adk-unit-guide 1ba92c9
PiperOrigin-RevId: 930802184

Author: joefernandez

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

@@ -0,0 +1,76 @@
+---
+name: adk-unit-guide
+description: Creates detailed code unit guides for source code documentation.
+---
+
+# ADK code unit guide
+This skill creates a detailed developer guide for new or updated code file or direct code input. The guide it generates is meant to explain the code to a developer who wants to use it in an application, but with a higher level of technical detail than what would appear in published developer documentation. Similar to a *unit test*, a *unit guide* provides generated, granular-level documentation for a unit of code, without worrying about bloating the actual developer documentation with too many details.
+
+## Input
+
+- Code files containing new functionality
+- Code unit tests (optional)
+- Code design files (optional)
+- Names of new methods and classes (optional)
+
+## Analysis
+
+- Review the code design files, if provided. Make note of:
+  - Purpose and intended use of the new or updated code units
+  - Classes that depend on the new or updated code units
+  - Additional dependencies required by the new or updated code units
+  - Limitations of the new or updated code units
+- Review specified code file for changes and named methods, if provided.
+- Determine what classes and code files may depend on the new or updated code units.
+
+## Output
+
+- Look for an existing guide in the `/docs/guides/***` directory of this repository.
+  - If a guide already exists, update the existing guide incrementally and prioritize preserving the previous content as much as possible.
+  - If no guide exists, create a guide file for the new code unit in the `/docs/guides/***` 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 guide in the location `/docs/guides/topic/function/class/index.md`.
+
+### Guide structure and content
+
+Use the following structure and instructions to create the guide for the code unit:
+
+```
+# Title: name of the code file or code unit
+
+- 2-sentence summary of the code unit
+
+## Introduction
+
+- Paragraph(s) explaining:
+  - The purpose and application of the code unit
+  - Key classes that depend on this code unit
+  - Developer problems solved by this code unit
+
+## Get started
+
+- Present a single, minimum implementation of the code unit to demonstrate its use.
+- Show enough of the containing classes to make it clear where the code could be used.
+- Use unit test code as a starting point for the code example, if available.
+
+## How it works
+
+- Explain how the code unit accomplishes its purpose or solves a problem.
+- Mention key code classes that depend on this code unit.
+- Mention code classes that this code unit depends on.
+- Explain any cross-class dependencies of the code unit.
+
+## Configuration options
+
+- If the code unit has configuration options, document them in a table detailing parameters, types, default values, and descriptions.
+
+## Advanced applications
+
+- Determine if there are advanced use cases for the code unit.
+- Add advanced applications of the code unit, including:
+  - Problem solved
+  - Implementations for special circumstances
+
+## Limitations
+
+- Mention any limitations of the code unit, if known.
+
+```