Add custom subagent support via markdown files

[Copilot]

Enables users to define specialized subagents through markdown files in $CODEX_HOME/agents/. Each agent runs as an isolated subagent with custom instructions, model selection, and sandbox policies.

Implementation

Core Infrastructure

• custom_agents.rs: Agent discovery from filesystem, frontmatter parsing (description, model, sandbox)
• CustomAgentTask: Executes agents via run_codex_conversation_one_shot with modified config
• Protocol: ListCustomAgents op, RunCustomAgent op with agent name + input items

Configuration via Frontmatter

---
description: "Expert code reviewer"
model: "claude-3-5-sonnet-20241022"
sandbox: "read-only"
---
You are an expert code reviewer...

Usage

// Discover available agents
conversation.submit(Op::ListCustomAgents).await?;

// Execute agent
conversation.submit(Op::RunCustomAgent {
    agent_name: "code-reviewer".to_string(),
    items: vec![UserInput::Text { text: "Review src/main.rs".into() }],
}).await?;

Security

• Sandbox policies: read-only (default), workspace-write, danger-full-access
• Inherits parent session feature flags and approval policies

Documentation

• docs/custom-agents.md: Feature documentation, best practices, troubleshooting
• docs/examples/custom-agents/: Four working examples (code-reviewer, documentation-writer, test-engineer, security-auditor)

Architecture

Follows existing subagent pattern (ReviewTask, CompactTask):

1. Load agent definition from $CODEX_HOME/agents/{name}.md
2. Spawn sub-conversation with custom base_instructions, model, sandbox_policy
3. Forward events to parent session
4. Complete on TaskComplete/TurnAborted

Agent names derived from filename stems. Only .md files discovered. Subdirectories not scanned.

Original prompt

增强本项目，让其支持可以通过编写md文件来自定义subagent。

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