Presence semantics

agent-presence counts agents that are actually working, not merely open terminal windows.

Presence state machine

SessionStart / UserPromptSubmit / PreToolUse / PostToolUse   -> running / heartbeat
Pi before_agent_start / turn_start / tool_execution_*         -> running / heartbeat
Stop / SessionEnd / session.idle / agent_end / shutdown      -> finished
No heartbeat for 3 minutes                                    -> expired
Expired + later live heartbeat                                -> running again
Laptop sleep / lid close / screen sleep                       -> reset to 0

finished is explicit and ignores late heartbeats. expired is TTL-inferred, so a later live heartbeat reopens the same session. Opening the Pi TUI alone is not counted — presence activates once you submit a task.