sprint-plan: producer feasibility gate (Phase 4) is skipped in lean/solo, contradicting Phase 2

**Problem.** Phase 2 states the producer feasibility gate (Phase 4) *runs first* and must complete before the file is written. But Phase 4 marks `'solo' → skip` and `'lean' → skip`, so in two of the three modes the plan reaches write approval **without** the feasibility review Phase 2 promises. **Expected vs. actual.** Expected: the gate runs before write approval in all modes (per Phase 2). Actual: it is bypassed in lean and solo. **Suggested fix.** Either run the gate unconditionally, or revise Phase 2's wording to acknowledge it is skipped in lean/solo and state where feasibility responsibility moves. **Location (links to the exact lines).** - [L52 — runs first](https://github.com/Donchitos/Claude-Code-Game-Studios/blob/984023ddac0d/.claude/skills/sprint-plan/SKILL.md#L52) - [L204 — Solo mode](https://github.com/Donchitos/Claude-Code-Game-Studios/blob/984023ddac0d/.claude/skills/sprint-plan/SKILL.md#L204) - [L205 — Lean mode](https://github.com/Donchitos/Claude-Code-Game-Studios/blob/984023ddac0d/.claude/skills/sprint-plan/SKILL.md#L205)