deferred tool registry and ToolSearch for large toolsets

[yai-dev]

Summary

Implement deferred tool loading and a ToolSearch path for large toolsets in the current architecture.

Background

As the plugin/capability ecosystem grows, eagerly injecting full schemas for every tool into every prompt becomes expensive and harms model accuracy.

Current Architecture Note

This should target the current @agentrail/app + @agentrail/capabilities split rather than the older host/tools package model.

Proposed Design

1. Introduce a deferred-tool registry in the current host/app layer
2. Expose a ToolSearch runtime tool that can fetch full schemas on demand
3. Defer eligible tool sources by default where prompt size would otherwise grow too large
4. Keep a clearly defined eager-tool allowlist for core execution tools

Affected Areas

• host/tool assembly in packages/app
• runtime tool/capability exports in packages/capabilities
• prompt/tool inventory construction paths

Acceptance Criteria

• Deferred tool names can be listed without full schemas in the prompt
• ToolSearch can return full schemas for selected/matched tools
• Core execution tools remain eager
• Integration tests prove a deferred tool can be searched and then called successfully
• No regression for agents that use only eager tools