Built-in multi-step custom modes for breaking down complex agent workflows

[digitarald]

Key scenarios:

• Plan: Plan implementation steps → Implement step by step → ✅
• Spec-Driven: Product Spec ↺ Design Plan ↺ Implementation Plan ↺ Implement → ✅
• Deep Architect: Draft Architecture ↺ Review Draft ↺ Implement → ✅
• TDD: Plan ↺ Red Phase (Failing Tests) → Green Phase (Make Tests Pass) → Refactor Phase (Improve Code) → ✅
• Low-code mockup flow for UX ideation: Brainstorm Solutions → Prototype Solutions → Iterate → Converge → ✅

Approach:

• v0: Baseline built-in plan/implement mode that can be easily customized to more specific engineering workflows
• Assume workflows can combine modes and prompts
• Combine with onboarding Chat modes/prompts: Add recommend config that informs in-chat suggestions for using modes and prompts #260212 to make sure users benefit from built-in flows and any team customizations.

[mubaidr]

When this is implemtned i belvie it can then help with: #261416

[joelvarty]

How will this work with MCP servers? Some MCP tool commands are "fetch" based and just help build out the context, but others are true "actions" with CRUD operations. I would want to know that only fetch tools are used in the planning phases, and CRUD operations would not happen until I want them to.