[FEATURE] Agent Teams should use existing agents

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

When using Agent Teams, lead agent spawns freshly-generated agents. But I already have agents configured that can be used instead

Proposed Solution

When spawning agent team teammates, the lead should discover and use matching agent definitions from .claude/agents/ before generating ad-hoc ones

Alternative Solutions

_No response_

Priority

Low - Nice to have

Feature Category

CLI commands and flags

Use Case Example

I have specialized agents in .claude/agents/ — backend-dev.md knows my TypeORM patterns and error handling, frontend-dev.md knows my React component library, reviewer.md has my security checklist.
When I use /agent backend-dev standalone, it works perfectly. But when I spawn an agent team (“create a backend teammate”), the lead ignores my existing agent files and creates a generic teammate instead.
Steps:

1. I ask the lead to spawn a backend teammate for implementing a feature
2. Lead creates a generic teammate that doesn’t know my project conventions
3. I have to either duplicate all agent knowledge into CLAUDE.md or paste it into spawn prompts

Expected: The lead should check .claude/agents/ for a matching agent definition and use it as the teammate’s base. If no match exists, then generate one ad-hoc like it does today.
This would make agent files reusable across all three contexts (standalone, sub-agent via context:fork, and agent teams) instead of maintaining the same role knowledge in two places.​​​​​​​​​​​​​​​​

Additional Context

_No response_