feat: reset idle assistant sessions after one hour

When a conversation's assistant session has been idle longer than
runtime.acp.assistantSessionResetIdleMs (default 1h, 0 disables) and
its owner has no unfinished proposals, the next inbound message starts
a fresh session instead of resuming. Binding lastActiveAt is persisted
per turn so the decision survives restarts.

config.example.json

@@ -48,6 +48,7 @@
       "promptTimeoutMs": 14400000,
       "cancelGraceMs": 5000,
       "idleTimeoutMs": 1800000,
+      "assistantSessionResetIdleMs": 3600000,
       "sweepIntervalMs": 60000,
       "maxProcesses": 8,
       "maxAssistantSessions": 4