all: make session lifecycle and dead-session recovery consistent

make daemon startup, live sessions, and persisted dead sessions
behave the same way in the cli, daemon, and protocol.

move live session coordination behind a per-session actor loop so
attach, detach, resize, and output fanout all go through one owner.

Author: seruman

README.md

@@ -82,7 +82,7 @@ default_scrollback = 10000
 # Persist dead session state to disk.
 state_persistence = true
 
-# Save session state every N seconds while the session is running.
+# Save session state every N seconds while the session is running. Must be > 0.
 state_persistence_interval = 30
 
 [client]