fix(task): neutralize custom subagent fallback description

[YumaKakuya]

Summary

• Replaces the misleading manual-only fallback for custom subagents with a neutral roles.md description.
• Adds test coverage for subagents without descriptions so custom roles do not imply false permission semantics.

Verification

• bun test test/tool/task.test.ts
• bun turbo typecheck

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