Function read_watermark 

pub fn read_watermark(project_hash: &str) -> Option<Watermark>

Read the watermark for the given project hash. Returns None when:

• the file doesn’t exist (first session on this repo),
• the file exists but is unreadable,
• the JSON fails to parse.

All three collapse into “treat this like a fresh repo” — the banner then shows everything learned to date, capped at the row limit.