Implement structured output across multiple agents and phases

[robgruen]

This pull request introduces a comprehensive "structured output" system for agent display content, enabling agents to return rich, semantically-typed documents (tables, lists, cards, images, etc.) with automatic markdown/text fallbacks for legacy clients. It adds a new StructuredContent type and related block types to the SDK, a builder function for creating structured results, and supporting helpers. The changes also include test scaffolding and documentation of the rollout plan and status.

The most important changes are:

Structured Output System (Core Types & Helpers):

• Added StructuredContent and a suite of typed block interfaces (e.g., TableBlock, ListBlock, CardBlock, etc.) to display.ts, allowing agents to describe UI output as a semantic block document with optional machine-readable payloads and automatic markdown/text alternates.
• Added the createStructuredResult helper to actionHelpers.ts for constructing action results with structured content, including automatic fallback and memory text derivation.
• Updated imports and exports throughout the SDK to support the new structured output system, including exporting block types and helpers. [1] [2]

Testing Infrastructure:

• Added a jest.config.cjs to the agent SDK package and new npm scripts in package.json to enable local and ESM-mode Jest testing for agent builders and derivation logic. [1] [2]

Documentation and Planning:

• Added a detailed status and rollout tracker for structured output in docs/plans/structured-output/STATUS.md, covering phases, agent adoption, test coverage, and open questions.

Other:

• Updated the repository to include the node_modules directory for the VSCode chat package.