Add Session Sharing (`/share` — export as markdown/gist)

[PureWeen]

Summary

The CLI's /share command exports a session as markdown, HTML, or a GitHub Gist for sharing with teammates. PolyPilot has no session export functionality.

CLI Behavior

• /share — export session conversation
• Formats: Markdown, HTML, GitHub Gist
• Includes conversation history, tool outputs, and code changes

Proposed Implementation

1. "Share" button in session context menu
2. Export format picker — Markdown / HTML / Gist
3. Markdown export — clean conversation transcript with code blocks
4. Gist export — create a GitHub Gist with the conversation (requires GitHub auth)
5. Copy to clipboard — quick share via paste
6. Share link — generate a shareable URL if using Gist

Priority: 🟢 LOW