Feature Request: Support custom agents from .claude/agents/ in Task tool

Feature Request

Summary

Allow the Task tool to use custom agent definitions from directory as options.

Current Behavior

The Task tool only supports three built-in agent types:

• general-purpose
• statusline-setup
• output-style-setup

Custom agent definitions can be created in .claude/agents/*.md (e.g., plan-architect.md, research-specialist.md), but cannot be invoked via the Task tool because they're not recognized as valid subagent_type values.

Desired Behavior

The Task tool should automatically discover and register custom agents from .claude/agents/:

# Example: .claude/agents/plan-architect.md
---
allowed-tools: Read, Write, Grep, Glob, WebSearch
description: Specialized in creating detailed, phased implementation plans
---

# Plan Architect Agent
[Agent instructions and behavioral guidelines...]

Then allow invocation:

Task {
  subagent_type: "plan-architect",
  description: "Create implementation plan",
  prompt: "[Task details]"
}

Use Case

Custom slash commands (e.g., /plan, /orchestrate) can reference specialized agents for workflow phases:

• plan-architect - Generates structured implementation plans
• research-specialist - Analyzes codebase and researches best practices
• code-writer - Executes implementation plans phase-by-phase
• debug-specialist - Root cause analysis and diagnostic reporting
• doc-writer - Updates documentation and generates summaries

This would enable better workflow orchestration with tool-restricted, purpose-specific agents while maintaining the existing agent definition format.

Current Workaround

Use subagent_type: "general-purpose" and include custom agent instructions in the prompt, but this loses the benefits of:

• Tool access restrictions per agent type
• Clear agent type discovery/documentation
• Separation of concerns between agent definitions and command logic

Proposed Implementation

1. Scan .claude/agents/*.md on startup
2. Parse frontmatter for allowed-tools and description
3. Register as valid subagent_type options for Task tool
4. Pass agent definition content as system instructions when invoked
5. Enforce allowed-tools restrictions from frontmatter

Additional Context

• Agent definitions already support frontmatter with allowed-tools and description
• Slash commands in .claude/commands/ already reference these agents
• This would align with the existing pattern of user-defined commands and hooks