Add `zenii session show <id>` command

[sprklai]

Description

Add zenii session show <id> to display session details and recent messages by calling GET /sessions/{id} and GET /sessions/{id}/messages.

Files to modify

• crates/zenii-cli/src/main.rs — add show action to Session subcommand

Acceptance criteria

• zenii session show <id> shows session metadata + last N messages
• --json flag for machine-readable output
• Graceful error for non-existent session ID
• Messages show role (user/assistant) and content preview

Difficulty: Easy (30-60 min)

This project is written in Rust. See CONTRIBUTING.md for setup instructions.