Align /simplify with a public slash entrypoint and multi-agent workflow

[yaohaowei0914]

Background

Rust already has bundled simplify prompt assets, but it does not yet match the public /simplify entrypoint and multi-agent behavior of the Bun reference.

Reference

• Entry: src/skills/bundled/simplify.ts
• Related modules: bundled skill registration and AgentTool
• Behavior: inspect the diff, spawn multiple review agents across reuse/quality/efficiency concerns, then drive fixes.

Rust Gap

• The simplify prompt already exists in src/skills/bundled.rs.
• Slash dispatch does not expose it as /simplify, and fork/agent execution is still incomplete.
• Suggested approach: separate the public entrypoint work from the deeper multi-agent execution work.

Acceptance

• Expose /simplify as a user-facing command.
• Reuse the multi-agent workflow when fork/agent support is available, with a fallback mode if not.
• Keep the output action-oriented rather than suggestion-only.
• Add tests for the slash->skill bridge.