fix(agent): throttle idle scans by default

docs/configuration.md

@@ -2159,7 +2159,7 @@ When a user is idle for longer than a configured threshold, nanobot **proactivel
   "agents": {
     "defaults": {
       "idleCompactAfterMinutes": 15,
-      "idleCompactCheckIntervalSeconds": 0
+      "idleCompactCheckIntervalSeconds": 60
     }
   }
 }
@@ -2168,12 +2168,12 @@ When a user is idle for longer than a configured threshold, nanobot **proactivel
 | Option | Default | Description |
 |--------|---------|-------------|
 | `agents.defaults.idleCompactAfterMinutes` | `15` | Minutes of idle time before auto-compaction starts. Set to `0` to disable. The default is close to a typical LLM KV cache expiry window, so stale sessions get compacted before the user returns. |
-| `agents.defaults.idleCompactCheckIntervalSeconds` | `0` | Minimum number of seconds between scans for idle sessions. |
+| `agents.defaults.idleCompactCheckIntervalSeconds` | `60` | Minimum number of seconds between scans for idle sessions. Set to `0` to scan on every idle tick (~1 s). |
 
 `sessionTtlMinutes` remains accepted as a legacy alias for backward compatibility, but `idleCompactAfterMinutes` is the preferred config key going forward.
 
 How it works:
-1. **Idle detection**: On each idle tick (~1 s), checks all sessions for expiration, subject to the minimum interval set by `idleCompactCheckIntervalSeconds`.
+1. **Idle detection**: On each idle tick (~1 s), checks whether an idle-session scan is due. By default, the full scan runs at most once per minute.
 2. **Background compaction**: Idle sessions summarize the older live prefix via LLM and keep the most recent legal suffix (currently 8 messages).
 3. **Summary injection**: When the user returns, the summary is injected as runtime context (one-shot, not persisted) alongside the retained recent suffix.
 4. **Restart-safe resume**: The summary is also mirrored into session metadata so it can still be recovered after a process restart.

nanobot/config/schema.py

@@ -155,7 +155,7 @@ class AgentDefaults(Base):
         serialization_alias="idleCompactAfterMinutes",
     )  # Auto-compact idle threshold in minutes (0 = disabled)
     idle_compact_check_interval_seconds: int = Field(
-        default=0,
+        default=60,
         ge=0,
     )  # Minimum interval in seconds between scans for idle sessions
     consolidation_ratio: float = Field(

tests/agent/test_auto_compact.py

@@ -180,10 +180,10 @@ class TestSessionTTLConfig:
         assert data["idleCompactAfterMinutes"] == 30
         assert "sessionTtlMinutes" not in data
 
-    def test_idle_scan_interval_defaults_to_zero(self):
+    def test_idle_scan_interval_defaults_to_sixty_seconds(self):
-        """The default should preserve a scan on every idle tick."""
+        """The config default should avoid scanning all sessions every idle tick."""
         defaults = AgentDefaults()
-        assert defaults.idle_compact_check_interval_seconds == 0
+        assert defaults.idle_compact_check_interval_seconds == 60
 
     def test_idle_scan_interval_uses_camel_case_config_key(self):
         """The JSON config should use the standard camelCase alias."""
@@ -226,8 +226,8 @@ class TestIdleScanThrottling:
 
         assert loop.auto_compact.check_expired.call_count == 2
 
-    def test_default_idle_scan_interval_checks_every_tick(self, tmp_path, monkeypatch):
+    def test_zero_idle_scan_interval_checks_every_tick(self, tmp_path, monkeypatch):
-        """The zero default should leave each idle tick eligible to scan."""
+        """An explicit zero should leave each idle tick eligible to scan."""
         monkeypatch.setattr("nanobot.agent.loop.time.monotonic", lambda: 1_000.0)
         loop = _make_loop(tmp_path)
         loop.auto_compact.check_expired = MagicMock()