fix(basic-memory): set default MCP project to avoid session errors

Claude Code sessions fail at startup when Basic Memory tools are called
without a project parameter. Since we only have one project ("main"),
set BASIC_MEMORY_MCP_PROJECT env var so the service defaults to it
without requiring each tool call to specify the project explicitly.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: stackptr

modules/nixos/llm/basic-memory.nix

@@ -42,6 +42,7 @@ in {
       environment = {
         HOME = dataDir;
         BASIC_MEMORY_CONFIG_DIR = dataDir;
+        BASIC_MEMORY_MCP_PROJECT = "main";
         UV_CACHE_DIR = "${dataDir}/.cache/uv";
       };