[Agent] Planning Agent

**A planning agent has been implemented. The planning agent has read-only tools except for one file that it can write in: a** `Plan.md` at the root of its workspace. **Missing features** * Switch from "Plan" to "Code" mode within a conversation. (probably CLI-only) * The user should be able to save their model preferences for "Plan" and "Code" mode. For example: * When in "Plan" mode, a developer may want to use GPT-5 for it's reasoning abilities * When in "Code" mode, a developer may prefer Claude Sonnet 4 for it's coding ability * Two planning modes * CLI-planning = light planning because the plan must be reviewable in the CLI * Deep planning. The plan can be precise and contain all main API signatures as well as the testing/feature validation strategy. The initial PLAN.md could be filled in with an empty template with sections (Current State / Implementation / Testing / …) ### If you find this feature request or enhancement useful, make sure to add a 👍 to the issue