Function to_plain 

pub fn to_plain(value: &Value) -> String

Convert a JSON Value into human-readable plain text.

Applies agent-first-data suffix-driven formatting:

• _ms → append ms, or convert to seconds if ≥ 1000
• _bytes → human-readable (446.1KB)
• _epoch_ms → RFC 3339
• _secret → ***
• Currency suffixes → formatted amounts