subagent_type with custom agents from .claude/agents/ returns 'Agent type not found'

Description

Custom agents defined in .claude/agents/ as plain markdown files were working as subagent_type values in the Agent tool until ~2026-04-07. As of v2.1.92, they fail with:

Agent type 'dev-engine' not found. Available agents: general-purpose, statusline-setup, Explore, Plan, claude-code-guide

Environment

• Claude Code version: 2.1.92
• OS: Windows 11 Pro (10.0.26200)
• Shell: bash (Git Bash)

Steps to Reproduce

1. Create .claude/agents/dev-engine.md with valid agent instructions (plain markdown, no YAML frontmatter — this format worked before)
2. Use the Agent tool with subagent_type: "dev-engine"
3. Error: "Agent type 'dev-engine' not found"

Agent files

$ ls -la .claude/agents/
dev-agent-design.md
dev-engine.md
dev-planner.md
dev-qa.md
dev-reviewer.md
quality-auditor.md
quality-enhancer.md

All files are directly in .claude/agents/ (not nested in subdirectories). They are plain markdown starting with # Agent Name headings.

Expected behavior

subagent_type: "dev-engine" should discover and use .claude/agents/dev-engine.md as the agent definition, as it did prior to this regression.

Actual behavior

Only the 5 hardcoded agent types are recognized: general-purpose, statusline-setup, Explore, Plan, claude-code-guide.

Notes

• No local update was made — this worked on 2026-04-07 and broke on 2026-04-08 without any claude CLI update
• This appears to be a server-side regression, not a local configuration issue
• This is not the same as #41715 (which covers /agents slash command UI and @ mention discovery — different code path)
• Workaround: spawn general-purpose agent and include the .claude/agents/*.md content in the prompt manually