harness: add hyperhive.autoCompact option (default true, false=disable proactive compaction)

nix/templates/harness-base.nix

@@ -228,6 +228,25 @@
     '';
   };
 
+  options.hyperhive.autoCompact = lib.mkOption {
+    type = lib.types.bool;
+    default = true;
+    description = ''
+      Enable proactive watermark-based compaction. When `true` (the
+      default) the harness automatically runs a notes-checkpoint turn
+      followed by `/compact` once the context window crosses 75% of
+      the model's limit, keeping later turns from hitting the hard
+      overflow path. Set to `false` to disable proactive compaction
+      entirely (`HIVE_COMPACT_WATERMARK_TOKENS=0`); the reactive path
+      (compact-on-overflow when the session is already past the limit)
+      still applies.
+
+      Disable for agents that run large-context models (sonnet/opus)
+      where the heuristic fires too early and discards useful history
+      before the session is actually close to the limit.
+    '';
+  };
+
   config = {
     environment.etc."hyperhive/extra-mcp.json".text = builtins.toJSON config.hyperhive.extraMcpServers;
 
@@ -260,6 +279,10 @@
     environment.variables = {
       HIVE_DEFAULT_MODEL = config.hyperhive.model;
       SHELL = "${pkgs.bashInteractive}/bin/bash";
+    } // lib.optionalAttrs (!config.hyperhive.autoCompact) {
+      # Zero watermark disables proactive compaction; the reactive path
+      # (compact-on-overflow) still fires when the session is truly full.
+      HIVE_COMPACT_WATERMARK_TOKENS = "0";
     };
 
     boot.isNspawnContainer = true;