feat: Python SDK feature parity — core types, hooks, tools, triggers, interactive loop

[codeitlikemiley]

Summary

Implements full feature parity between the Rust and Python Antigravity SDKs, closing all identified gaps from the SDK comparison analysis.

Changes

New Modules (5 files)

File	Description
src/error.rs	Typed error hierarchy: AntigravityError (Connection, Execution, Validation)
src/context.rs	Hierarchical HookContext state store (Session→Turn→Operation)
src/tool_context.rs	Session-scoped ToolContext with state store and agent communication
src/trigger_helpers.rs	every() periodic trigger factory
src/interactive.rs	run_interactive_loop() REPL utility (native only)

Extended Modules (4 files)

File	Changes
src/hooks.rs	Added on_session_end, post_turn, on_compaction hooks + dispatchers + tests
src/tools.rs	Added needs_context() / call_with_context() for context-aware tools
src/types.rs	Added multimodal pipeline (Media, Content, MimeType — 32 extensions), trigger types
src/lib.rs	Wired all new modules into public API

Documentation

File	Changes
README.md	Added 7 new sections: Error Handling, Multimodal Content, Lifecycle Hooks, Hook Context, Context-Aware Tools, Trigger Helpers, Interactive Loop

Python SDK Feature Mapping

Python Feature	Rust Implementation	Status
AntigravityConnectionError / AntigravityValidationError	AntigravityError::Connection / AntigravityError::Validation	✅
Image, Document, Audio, Video, Content	Media, Content, MimeType enums	✅
SessionContext → TurnContext → OperationContext	HookContext::new() / HookContext::child()	✅
ToolContext with get_state/set_state	ToolContext struct	✅
on_session_end, post_turn, on_compaction	Hook trait methods	✅
every() trigger helper	trigger_helpers::every()	✅
run_interactive_loop()	interactive::run_interactive_loop()	✅

Testing

• 90 unit tests — all pass
• 4 integration tests — all pass
• 4 doc-tests — all pass (including new modules)
• cargo clippy --all-targets -- -D warnings — 0 warnings

Backwards Compatibility

All changes are fully backwards compatible:

• New hook methods have default no-op implementations
• Tool::needs_context() defaults to false
• Existing PolicyEnforcer uses defaults automatically
• No breaking changes to public API