Call this immediately after reading a file for the first time. Must be called BEFORE check_staleness can detect changes to that file. Records current mtime and content hash.

Check whether a previously-stamped file has changed since you last read it. If no prior stamp exists it will tell you to stamp first. Call this before editing any file you stamped earlier — if stale, re-read it.

How long has this session been running and how many messages have been exchanged. Useful for understanding context age.

Manually mark all events from a source as stale. Used when you know a resource has changed and want to ensure stale warnings fire.