Shell environment variables

Every terminal Saggar starts is a normal login shell with these additional values:

Variable	Value
SAGGAR	1
TERM_PROGRAM	saggar
SAGGAR_SESSION	A unique identifier for the terminal session

Prompts and tools can use SAGGAR=1 or TERM_PROGRAM=saggar to detect the app. Provider hooks and the saggar CLI use SAGGAR_SESSION to address the calling terminal without guessing.