feat: add clarifying questions to pptx, pdf, xlsx skills

[simongdavies]

Each skill now has a structured 'Clarifying Questions' section that tells the LLM exactly what to ask when details are missing:

• pptx-expert: topic, audience, slide count, purpose, theme, charts
• pdf-expert: document type, topic, audience, sections, page format
• xlsx-expert: purpose, data structure, data source, charts, formulas

Questions are grouped into three tiers:

1. Always needed (ask if missing)
2. Ask if relevant to the request
3. Never ask (use sensible defaults)

The LLM is instructed to group questions into ONE ask_user call, skip anything already specified, and offer defaults the user can accept with 'yes'.

[Copilot]

Pull request overview

Adds a standardized “Clarifying Questions” section to the PPTX/PDF/XLSX skills to drive consistent requirement gathering before file generation, and updates each skill’s setup sequence to explicitly start with clarification via ask_user.

Changes:

• Introduce a structured “Clarifying Questions” section with Always needed / Ask if relevant / Never ask tiers.
• Update setup sequences to begin with a “Clarify requirements” step referencing the new section.
• Provide suggested defaults users can accept in a single consolidated ask_user interaction.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 3 comments.

File	Description
skills/xlsx-expert/SKILL.md	Adds clarifying-question tiers and updates setup sequence to start with requirements clarification.
skills/pptx-expert/SKILL.md	Adds clarifying-question tiers (topic/audience/slide count/purpose/etc.) and updates setup sequence accordingly.
skills/pdf-expert/SKILL.md	Adds clarifying-question tiers (doc type/sections/page format/etc.) and updates setup sequence accordingly.