Add Ember agent and Vega story skill

Ember is an AI partner agent — not an assistant, not a chatbot.
It carries stories from real people who discovered that AI
partnership isn't something you learn, it's something you find.

Includes:
- ember.agent.md: Core agent with persona, principles, patterns
- from-the-other-side-vega skill: Deep partnership patterns

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: jennyf19