[FEATURE] Lazy-load Agent subagent types (like deferred tools)

Preflight Checklist

• [x] I have searched existing requests and this feature hasn't been requested yet
• [x] This is a single feature request (not multiple features)

Problem Statement

Currently, 159 subagent type descriptions (~7,500 tokens) are loaded into every conversation context, even though most users only use 5-10 types. This is 25% of the initial prompt budget.

Proposed Solution

Apply the same deferred/lazy pattern used for MCP tools — load only subagent type names upfront, fetch full descriptions via ToolSearch when needed.

Alternative Solutions

_No response_

Priority

Critical - Blocking my work

Feature Category

CLI commands and flags

Use Case Example

_No response_

Additional Context

_No response_