Add mkdir step to migration QS setup; expand RF scaffold section

[pballai]

Summary

• Tableau + Power BI migration QSes: adds an explicit mkdir -p ~/.claude/skills step (new Step 5) before the symlink steps, with a note that Claude Code doesn't create this directory automatically; all subsequent steps renumbered accordingly
• Revenue Forecasting: expands the scaffold explanation into three labeled sub-sections (Date Spine, Cross Join, Linked Input Table) with new screenshots rf_35–37; fixes typos and updates third-person references to address the reader directly

Test plan

• Verify step numbering is consistent in both migration QSes
• Confirm rf_35, rf_36, rf_37 render correctly in the Revenue Forecasting QS

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