[Feature]: /share command to upload session as secret GitHub gist

[kowyo]

Feature description

A /share command that uploads the raw session data (system prompt, tool definitions, and the full conversation) as a secret GitHub gist, producing a shareable URL.

Problem it solves

When debugging agent behavior or showing a conversation to others, there is currently no built-in way to share a session. Users have to manually locate the JSONL file in ~/.mini-agent/sessions/, copy it, and find a way to share it. This is cumbersome, especially for non-technical recipients who just need to see what happened in the conversation.

A single command that produces a URL would make sharing trivially easy.

Proposed solution

Add a /share command in interactive mode that:

• Packages the current session into a portable format containing the system prompt (with tool definitions and any loaded skills/context), followed by the full message history.
• Uses the gh CLI (gh gist create) to upload it as a secret GitHub gist.
• Prints the resulting gist URL in the terminal.

The shared content should be plain text (e.g., Markdown), not HTML or CSS.

Additional context

• Requires gh CLI to be installed and authenticated (gh auth status).
• The command should show a loading indicator while uploading.
• If gh is not installed or not logged in, print a clear error message.