Add `--quiet` global flag to CLI

[sprklai]

Description

Add a global --quiet / -q flag that suppresses non-essential output (banners, status messages, spinners). Only the core data result is printed to stdout.

Files to modify

• crates/zenii-cli/src/main.rs — add global --quiet flag and suppress decorative output when set

Acceptance criteria

• zenii -q memory search "query" only outputs results, no banners
• Errors still print to stderr even in quiet mode
• Combines with --json for clean scripting: zenii -q --json provider list

Difficulty: Easy (30-60 min)

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