chore: update agents.md to include versioning strategy

Adds a description for docstring and comments in the AGENTS.md and adds a section for Versioning that describes how ADK follows Semantic Versioning 2.0.0

PiperOrigin-RevId: 778667398

Author: hangfei

AGENTS.md

@@ -112,3 +112,88 @@ Run below command:
 ```bash
 $ pytest tests/unittests
 ```
+
+## Docstring and comments
+
+### Comments - Explaining the Why, Not the What
+Philosophy: Well-written code should be largely self-documenting. Comments
+ serve a different purpose: they should explain the complex algorithms,
+ non-obvious business logic, or the rationale behind a particular implementation
+ choice—the things the code cannot express on its own. Avoid comments that
+ merely restate what the code does (e.g., # increment i above i += 1).
+
+Style: Comments should be written as complete sentences. Block comments must
+begin with a # followed by a single space.
+
+## Versioning
+ADK adherence to Semantic Versioning 2.0.0
+
+Core Principle: The adk-python project strictly adheres to the Semantic
+Versioning 2.0.0 specification. All release versions will follow the
+MAJOR.MINOR.PATCH format.
+
+### Breaking Change
+
+A breaking change is any modification that introduces backward-incompatible
+changes to the public API. In the context of the ADK, this means a change that
+could force a developer using the framework to alter their existing code to
+upgrade to the new version. The public API is not limited to just the Python
+function and class signatures; it also encompasses data schemas for stored
+information (like evaluation datasets), the command-line interface (CLI),
+and the data format used for server communications.
+
+### Public API Surface Definition
+
+The "public API" of ADK is a broad contract that extends beyond its Python
+function signatures. A breaking change in any of the following areas can
+disrupt user workflows and the wider ecosystem of agents and tools built with
+ADK. The analysis of the breaking changes introduced in v1.0.0 demonstrates the
+expansive nature of this contract. For the purposes of versioning, the ADK
+Public API Surface is defined as:
+
+- All public classes, methods, and functions in the google.adk namespace.
+
+- The names, required parameters, and expected behavior of all built-in Tools
+ (e.g., google_search, BuiltInCodeExecutor).
+
+- The structure and schema of persisted data, including Session data, Memory,
+ and Evaluation datasets.
+
+- The JSON request/response format of the ADK API server(FastAPI server)
+ used by adk web, including field casing conventions.
+
+- The command-line interface (CLI) commands, arguments, and flags (e.g., adk deploy).
+
+- The expected file structure for agent definitions that are loaded by the
+ framework (e.g., the agent.py convention).
+
+#### Checklist for Breaking Changes:
+
+The following changes are considered breaking and necessitate a MAJOR version
+ bump.
+
+- API Signature Change: Renaming, removing, or altering the required parameters
+ of any public class, method, or function (e.g., the removal of the list_events
+ method from BaseSessionService).
+
+- Architectural Shift: A fundamental change to a core component's behavior
+ (e.g., making all service methods async, which requires consumers to use await).
+
+- Data Schema Change: A non-additive change to a persisted data schema that
+ renders old data unreadable or invalid (e.g., the redesign of the
+  MemoryService and evaluation dataset schemas).
+
+- Tool Interface Change: Renaming a built-in tool, changing its required
+ parameters, or altering its fundamental purpose (e.g., replacing
+ BuiltInCodeExecutionTool with BuiltInCodeExecutor and moving it from the tools
+ parameter to the code_executor parameter of an Agent).
+
+- Configuration Change: Altering the required structure of configuration files
+ or agent definition files that the framework loads (e.g., the simplification
+ of the agent.py structure for MCPToolset).
+
+- Wire Format Change: Modifying the data format for API server interactions
+ (e.g., the switch from snake_case to camelCase for all JSON payloads).
+
+- Dependency Removal: Removing support for a previously integrated third-party
+ library or tool type.