Antigravity SDK 0.1.21

[Improvements] Experimental API isolation with beta namespaces: Introduces the @beta decorator and google.antigravity.beta module, enabling access to preview and experimental features on class and instance namespaces without polluting the stable API surface.

[Improvements] Declarative subagent skills configuration: Adds skills_config support to SubagentConfig, allowing developers to explicitly configure subagent skill inheritance, override skills, or isolate subagents from parent skills.

[Improvements] Bulk hook registration: Enables registering sequences of hooks at once during Agent session initialization and within HookRunner.

[Improvements] Beta API decorator and namespace: Added @beta and BetaNamespace support to expose preview classes, methods, and properties under dedicated .beta accessors and google.antigravity.beta.

[Improvements] Subagent skills management: Added SubagentSkillsConfig, SubagentInheritSkillsConfig, SubagentNoneSkillsConfig, and SubagentOverrideSkillsConfig to SubagentConfig for declarative control over subagent skills.

[Improvements] Session hook collections: Updated HookRunner and Agent session initialization to accept collections of hooks in addition to individual registrations.

[Improvements] Qualified typing in tool annotations: Expanded ToolRunner string annotation inspection to resolve qualified typing wrappers (typing.Optional, typing.Union, and typing.Annotated) when injecting ToolContext.

[Improvements] UsageMetadata arithmetic: Harmonized UsageMetadata.__sub__ typing and zero-value identity behavior to align with UsageMetadata.__add__.

[Improvements] Tool inspection: Added __repr__ to ToolWithSchema for clearer terminal output and debugging inspection.

[Fixes] Tool call deduplication: Fixed an issue in Conversation.receive_chunks() where tool calls with empty string IDs (id='') were treated as duplicate IDs and dropped; they are now treated as absent IDs and yielded correctly.