Guide the user through MVP-mode planning for a phase. The command:

1. Prompts for an "As a / I want to / So that" user story (three structured questions)
2. Runs SPIDR splitting check — if the story is too large, walks through Spike/Paths/Interfaces/Data/Rules and offers to split into multiple phases
3. Writes **Mode:** mvp and the reformatted **Goal:** to the phase's ROADMAP.md section
4. Delegates to /gsd plan-phase <N> which auto-detects MVP mode via the roadmap field

Phase 1 of the vertical-mvp-slice PRD shipped the planner-side machinery; this command is the user entry point for it.

<execution_context> @/.claude/gsd-core/workflows/mvp-phase.md @/.claude/gsd-core/references/spidr-splitting.md @~/.claude/gsd-core/references/user-story-template.md </execution_context>

<runtime_note> Copilot (VS Code): Use vscode_askquestions wherever this workflow calls AskUserQuestion. Equivalent API. </runtime_note>

Phase number: $ARGUMENTS (required — integer or decimal like `2.1`)

The phase must already exist in ROADMAP.md (created via /gsd new-project, /gsd add-phase, or /gsd insert-phase). This command does not create new phases — it converts an existing phase to MVP mode.

Execute the mvp-phase workflow from @~/.claude/gsd-core/workflows/mvp-phase.md end-to-end. Preserve all gates: phase existence, status guard (refuse in_progress/completed), user-story format validation, SPIDR splitting check, ROADMAP write confirmation, plan-phase delegation.