fix(haivemind): clear blank Textual Select

Textual 8 treats Select.BLANK as False, causing ConfigScreen to crash.

.beads/issues.jsonl

@@ -1,3 +1,4 @@
+{"_type":"issue","id":"nix-overlay-dv3","title":"Fix Config screen crash on F2","description":"F2 opens ConfigScreen then Textual raises InvalidSelectValueError because _refresh_aggregator_dropdown assigns Select.BLANK, resolved as False, instead of clearing selection.\\n\\nAcceptance: Config screen opens with saved consensus configuration; regression covers no selected aggregator; targeted tests pass.","status":"in_progress","priority":1,"issue_type":"bug","assignee":"Alexander Miroshnichenko","owner":"alex@millerson.name","created_at":"2026-08-20T11:34:03Z","created_by":"Alexander Miroshnichenko","updated_at":"2026-08-20T11:34:07Z","started_at":"2026-08-20T11:34:07Z","dependency_count":0,"dependent_count":0,"comment_count":0}
 {"_type":"issue","id":"nix-overlay-a9m","title":"Fix haivemind startup after MainScreen mount","description":"Restore deferred engine startup while preserving MainScreen widget lookup fix.","status":"closed","priority":1,"issue_type":"bug","assignee":"Alexander Miroshnichenko","owner":"alex@millerson.name","created_at":"2026-08-20T11:20:36Z","created_by":"Alexander Miroshnichenko","updated_at":"2026-08-20T11:21:43Z","started_at":"2026-08-20T11:20:43Z","closed_at":"2026-08-20T11:21:43Z","close_reason":"Restored engine startup after MainScreen mount and verified package build.","dependency_count":0,"dependent_count":0,"comment_count":0}
 {"_type":"issue","id":"nix-overlay-q00","title":"Add haivemind package","description":"Package https://github.com/dev-boz/haivemind in the Nix overlay, document it, validate the build, and complete repository workflow.","status":"closed","priority":2,"issue_type":"task","assignee":"Alexander Miroshnichenko","owner":"alex@millerson.name","created_at":"2026-08-08T16:32:37Z","created_by":"Alexander Miroshnichenko","updated_at":"2026-08-08T16:36:41Z","started_at":"2026-08-08T16:33:00Z","closed_at":"2026-08-08T16:36:41Z","close_reason":"Closed","dependency_count":0,"dependent_count":0,"comment_count":0}
 {"_type":"issue","id":"nix-overlay-67e","title":"Add traycer package","description":"Add new package for traycer from https://github.com/traycerai/traycer. Steps: research repo build system, create packages/traycer/, stage with git add, test nix build, update README.","status":"closed","priority":2,"issue_type":"task","assignee":"Alexander Miroshnichenko","owner":"alex@millerson.name","created_at":"2026-08-07T08:59:25Z","created_by":"Alexander Miroshnichenko","updated_at":"2026-08-07T09:22:07Z","started_at":"2026-08-07T09:00:29Z","closed_at":"2026-08-07T09:22:07Z","close_reason":"Traycer package added, built, pushed","dependency_count":0,"dependent_count":0,"comment_count":0}

packages/haivemind/package.nix

@@ -24,7 +24,15 @@ python3Packages.buildPythonApplication rec {
     textual
   ];
 
-  patches = [ ./fix-chat-log-race.patch ];
+  patches = [
+    ./fix-chat-log-race.patch
+  ];
+
+  postPatch = ''
+    substituteInPlace haivemind_tui.py \
+      --replace-fail "sel.value = Select.BLANK" "sel.clear()" \
+      --replace-fail "v is Select.BLANK" "v is Select.NULL"
+  '';
 
   doCheck = false;
   pythonImportsCheck = [