Add xls-formatting-creator meta-skill

[alexa-perlov]

Summary

• Adds /xls-formatting-creator slash command and companion meta-skill under financial-analysis/, mirroring the existing /ppt-template + ppt-template-creator pattern for Excel.
• Takes one or more representative .xlsx files, extracts house-style formatting (fonts, fills, borders, number formats, column widths, named styles, theme colors), and generates a self-contained skill that encodes the conventions.
• Headline feature: the color-coding extractor classifies each cell as numeric_input / text_label / formula / intra_link / external_link and tabulates modal font color per class, so conventions like blue-input / black-formula / green-cross-sheet-link get surfaced automatically rather than hand-documented.

What's in the generated skill

Each generated skill contains:

• assets/reference.xlsx — one of the representative input workbooks, for future-reference when rules are ambiguous.
• SKILL.md — self-contained: house color-coding rules, font table, number format strings, border conventions, column widths, layout conventions, and ready-to-use code paths for both openpyxl and Office JS.

Test plan

• Install the marketplace locally (/plugin marketplace add <repo> → /plugin install financial-analysis) and invoke /xls-formatting-creator <path.xlsx> against a real operating model or LBO output.
• Confirm the extractor reports the correct modal font color for inputs, formulas, and cross-sheet links before encoding it in the generated skill.
• Run the generated skill to build a new schedule and verify it looks house-compliant when opened in Excel.
• Try against a workbook using a non-classic convention (e.g., red for inputs) to confirm the meta-skill adapts rather than hardcoding the banker default.