feat: add handoff and write-a-skill skills

[futuregerald]

Adds two productivity skills:

• handoff: Compact conversations into handoff docs for agent continuity across sessions. Saves to OS temp dir, includes suggested skills for the next agent, and integrates with Prism session memory.
• write-a-skill: Guide for creating new skills with proper structure, progressive disclosure, and bundled resources. Covers SKILL.md template, description rules, file splitting heuristics, and review checklist.